All users may post questions about MCAT, DAT, OAT, or PCAT general chemistry here. We will answer the questions as soon as we reasonably can. If you would like to know what general chemistry topics appear on the MCAT, you should check the MCAT Student Manual (http://www.aamc.org/students/mcat/studentmanual/start.htm)

Acceptable topics:
-general, MCAT-level gen chem.
-particular MCAT-level gen chem problems, whether your own or from study material
-what you need to know about gen chem for the MCAT
-how best to approach to MCAT gen chem passages
-how best to study MCAT gen chem
-how best to tackle the MCAT physical sciences section

Unacceptable topics:
-actual MCAT questions or passages, or close paraphrasings thereof
-anything you know to be beyond the scope of the MCAT

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If you really know your gen chem, I can use your help. If you are willing to help answer questions on this thread, please let me know. Here are the current members of the General Chemistry Team:

-QofQuimica (thread moderator): I have my M.S. in organic chemistry and I'm currently finishing my Ph.D., also in organic chemistry. I have several years of university general chemistry TA teaching experience. In addition, I teach general chemistry classes through Kaplan for their MCAT, DAT, OAT, and PCAT courses. On the MCAT, I scored 14 on PS, 43 overall.

-Learfan: Learfan has his Ph.D. in organic chemistry and several years worth of industrial chemistry experience. He scored 13 on the PS section of the MCAT, and 36 overall.

-Sparky Man: Sparky Man has his Ph.D. in physical chemistry. He scored 14 on the PS section of the MCAT, and 36 overall.

-GCT: GCT scored in the 99th percentile on the PCAT. He has also taught introductory physics and general chemistry.

Question: Is it ok to calculate the emf just as if the cell were galvanic, and then simply switch to a negative value?

Yes, that will work as long as you write one half-reaction as a reduction and the other as an oxidation. Many times both half-reactions will be written as reductions, so check where the electrons are in the equations. If they are on the left side of the equation, it's a reduction half-reaction, and if they are on the right side of the equation, it's an oxidation half-reaction. You must always have one of each in any redox reaction, because if something is getting reduced than something else must be getting oxidized.

Question: for standard cell potential: which formula is correct: Eo = Ereduction + Eoxidation? Or Eo(cell) = Eo(cathode) - Eo(anode)? How can this problem be solved?

Consider the following electrode potentials:

Cu2+ + 2e- --> Cu(s) Eo = +0.34 V
2H20 --> O2 + 4H+ + 4e- Eo = -1.23 V

What is the Eo(cell) for the reaction shown in the following equation?

2Cu2+ + 2H20 --> 2Cu(s) + O2 + 4H+

A) -0.89 V
B) +0.55 V
C) +1.57 V
D) + 1.91 V

My advice to you is to not use either version of the formula, as it will only confuse you. (Guess I didn't need to tell you that, right? ) Instead, what you should do is attack any electrochemistry problem by first considering the following:

What is the form in which the two half-cell reactions are written? Generally, but not always, you will see both half-reactions written as reduction potentials. You can determine this because you will see that the electrons are being added on the left side of the equation. Remember, when you add electrons, it's a reduction. Conversely, if you lose electrons, it's an oxidation, and you will see the electrons on the right side of the equation.

If both half-reactions are written as reductions, you must turn one of the them backward into an oxidation half-reaction. (If something is being reduced, something else better be getting oxidized because those electrons have to come from somewhere.) The tough part is deciding which half-reaction to turn backward. If you have a galvanic cell (most common case), you will want to turn the half-reaction with the smaller reaction potential backward. (Don't forget that a number like -1.2 would be smaller than one like -0.3). You do this because galvanic cells will always have a positive Ecell (they have to, because they are spontaneous, and the equation that relates G with Ecell has a minus sign in it: G=-nFEcell). If the cell is electrolytic, then it is not spontaneous, Ecell should be negative (making G positive) and you will turn the half-reaction with the larger reaction potential backward instead. Remember, in either case, when you turn the half-reaction backward, you change the sign of that half-reaction's reaction potential.

Ok, so now let's consider your specific example. The first half-reaction (the Cu one) is a reduction half-reaction. Again, I know that because the electrons are being added on the left side; Cu is gaining electrons. The second one, in contrast, is an oxidation half-reaction. The electrons are on the right side; O is losing electrons. Good, so in this case, nothing needs to be turned around, because we already have one reduction and one oxidation. You can see that this cell is an electrolytic one as written. (Add the two reaction potentials together, and you are going to get a negative number for Ecell) Looking at the third equation that combines both half-reactions, you can see that it is written in the same fashion as the two half-reactions above it: Cu is getting reduced, and O is getting oxidized. Thus, again, you have an electrolytic cell, nothing needs to be turned backward because one partner is oxidizing and one is reducing, and you merely need to add the two reaction potentials together to get your -0.89.

Really, there isn't any need to do any math on this problem. As soon as you recognize that you have an electrolytic cell, there is only one possible right answer for this question, because the others are all positive for Ecell.

Remember that you do not have to multiply the values of the reaction potentials by the number of moles of electrons. The reason why is that reaction potential is an intrinsic property; it is independent of the amount of material you have. That is, one gram of Cu has the same reaction potential as 1 kg of Cu does. (Temperature and density are other examples of intrinsic properties.) The properties where you do have to multiply by the number of moles are extrinsic properties; those do depend on the amount of material. For example, when you use Hess's law, it matters greatly how much material you have because the more compound you have, the more heat it will give off. (Energy and volume are some other examples of extrinsic properties.)

Here is SilvrGrey330's extremely clever mnemonic to remember salt solubility rules.

C A S H n Gia

Read it as "Cashin' Gia"...how to remember that? well the story is...im a pimp...and gia is my hoe, and i need to get my cash from her. hence...Cashing from gia.

C is clorates, A is acetates, S is sulfates, H is halogens, n is Nitrates, and Gia is Group I A metals. ---> THESE ARE ALL SOLUBLE, XCEPT

for S: Ca, Ba, Sr.....just remember the tv network CBS
for H: Ca, Ba, Sr + Happy...whats happy? Hb Ag Pb ...mercury, silvr and lead...add a py to the end and all the first letters spell HAPPY

and if its not part of CASHnGIA...its insoluble.

You might not be able to figure out all of them (most transition metals can have more than one cation), but you can make a good educated guess if you pick whichever ion will give that element either a half full (d5) or a completely full (d10) subshell. So, for Fe, you can see that it has 8 valence electrons (2 s's and 6 d's), and it will lose its 4s electrons first. It would like to have a 3d5 configuration, which is why Fe3+ is a common ion. Fe2+ can also occur because it would leave 6 electrons, so that both the s and the d's can be half full.

Acidic compounds are either proton donors (Bronsted-Lowry and Arrhenius definitions) or lone pair acceptors (Lewis definition). So for acids, you should look for protons that are attached to electronegative atoms, or else other elements that are electron-deficient. Basic compounds can either be hydroxyl donors (Arrhenius definition), proton acceptors (Bronsted-Lowry definition), or lone pair donors (Lewis defintion). So for bases, you should look for hydroxyl groups, or else atoms having lone pairs on them, especially if they are negatively charged.

All MCAT students should memorize this list of strong acids and strong bases. Any acid not on this list should be considered to be a weak acid. Most other bases are also weak bases, but realize that there are also some strong organic bases. I have listed some examples.

Strong Acids:
-H2SO4
-HNO3
-HCl
-HBr
-HI
-HClO3
-HClO4
*note that HF is NOT a strong acid, and its omission from the list is not an accident!!!
**only the first proton on H2SO4 is strong (i.e., completely dissociates); the second one is weak.

Strong Bases:
-Group I elements with hydroxide (ex. NaOH, KOH)
-Group II elements with hydroxide [ex. Ca(OH)2]
-some organic bases, including alkoxide ions, sodium hydride, Grignard reagents, and LDA

(The poster calculated that the answer was 15 mL, but the correct answer is 30 mL. Here is why.)

You are confusing two types of problems: dilution problems, where you need to find the difference between the total volume and the starting volume to get the volume added to make the diluted solution; and titration problems, such as what you are doing here. Remember, for titrations the volumes and molarities are only relevant in that we are using them to get the number of MOLES of acid and base, which must be equal at the titration's end point. You can get the number of moles of each by multiplying their molarities times their volumes. First you multiply 15 mL HNO3 x 1 mmol/mL as you did, to get 15 mmol. So you then know that your 0.5 mmol/mL concentration of KOH must also have the same number of moles as the acid, and you divide 15/0.5 to get 30 mL of base. That is the correct answer.

Stop a minute and rationalize this to yourself. You have a base solution with a concentration that is half of the acid's concentration. Both your acid and base are monoprotic and monobasic (important to consider, because you'd have to use twice as much base to neutralize the same amount of 1M H2SO4). So you would logically expect that you need twice as much of your base to neutralize that amount of acid. And this is indeed the case.

One piece of advice: try to avoid the urge to plug and chug when solving PS problems. Always start by considering 1) what is the problem asking for, and 2) what kind of answer is reasonable for this problem? If you have a good grasp of where you want to go and a ballpark idea of what a reasonable answer should be, it will help you avoid making silly mistakes on the MCAT due to miscalculation. Also, if you can avoid doing calculations altogether, that will save you time as an added bonus. This problem, for example, can be solved in a few seconds once you realize that your base's volume should be twice the acid's volume, as I explained above.

Question: When a cell is put in distilled water, the water will flow into the cell (from hypotonic to hypertonic). So where is osmotic pressure high, inside the cell or outside? Based on the formula of osmotic pressure, it should be inside the cell (I mean this formula: osmotic pressure = R* Molarity* Temperature). Do I think correctly?

Osmotic pressure is the pressure that must be applied to your cell membrane to PREVENT the water from coming inside when you place the cell in the hypotonic solution. It's a way to measure how great the difference in concentration is on each side of the membrane; if you have a really concentrated cell in a really dilute solution, you'll need a lot more pressure to stop that water from coming in versus if the two concentrations are closer to each other. If you put the cell into a hypertonic solution, you have the reverse situation. Now, the osmotic pressure is equivalent to the pressure necessary to stop the water from leaving the cell to go into the more concentrated solution surrounding it.

You can use the osmotic pressure equation to calculate what the osmotic pressure is for a given solution, such as 20 g of NaCl dissolved in 500 mL of water at STP. So if your cell is hypertonic to your solution, as in your example, then yes, the osmotic pressure is greater inside the cell versus outside. The hypertonic solution will always have a higher osmotic pressure because it has a higher molarity, and the temperature and gas constant are both constant. Don't forget to convert your temperature to Kelvins, not Celsius, and remember that salts dissociate into ions, so that 1 mole of NaCl is actually 2 moles (approximately) of ions.

STP is used for the ideal gas law, and it is 273K, 1 atm of pressure. Memorize that 1 mole of ideal gas at STP occupies 22.4 L.

Standard conditions is used for thermodynamics problems, and it is 298K, 1 atm pressure, 1 M concentrations of all compounds.

One helpful hint for doing general chemistry questions on the MCAT: always remember to convert your temperature to Kelvins. The only time this doesn't matter is if you have a delta T, since the size of a Kelvin is the same as the size of a Celsius degree and you are taking the difference in that case. But if there isn't a delta, you must convert to K.

Question: how would you calculate 10^(.1) by hand on the mcat? or 10^(.2)?

You should never run into these kinds of calculations on the MCAT; they know that you don't have a calculator, and no one expects you to know the tenth or fifth roots of numbers! If by some bizarre chance you did get a problem like that, you should try to estimate the answer by bracketing it. For example, I know that 1^10 = 1, and 2^10 = 1024. So the tenth root of 10 will be a number between 1 and 2, closer to 1 than to 2. (maybe 1.1 ish?) 10^.2 is the same thing as the fifth root of 10, which would also be a number between 1 and 2 (1^5 = 1 and 2 ^5 = 32, so I'd guess something like 1.5 ish). In general, never waste time on the MCAT doing exact calculations. Round and take short cuts with the math as much as you possibly can.

Question: Did anyone who previously took the Mcat have to use the quadratic formula for solving a pH problem where the denominator x could not be discarded???

Goodness, no! The testmaker IS aware that you don't have a calculator and that the test is timed! Take shortcuts with the math whenever possible.

How do i get density of a certain gas using PV=nRT ?

when is R = 8.314.... and when is it = 0.0821....

How do i get density of a certain gas using PV=nRT ?

You will need the temperature, the pressure, and the molecular weight (MW) of the gas. By definition,

d=(m/V)

and

n=(m/MW)

Substituting the second definition into the ideal gas law, we get

PV=(mRT)/MW

Rearranging the equation above, we get

(m/V)=(P x MW)/(RT)

So,

d=(P x MW)/(RT)

when is R = 8.314.... and when is it = 0.0821....

The value of R depends on the units that you are using for pressure. If you measure your pressure in atm, then R=0.0821 (L x atm)/(mol x K). If you measure pressure in Pa, then R=8.314 J/(mol x K). You do not need to memorize the values of R for the MCAT.

Im wondering if i have this correct The 4s subshell fills before the 3d subshell, but electrons get removed from the 4s subshell before the D subshell?

Im wondering if i have this correct The 4s subshell fills before the 3d subshell, but electrons get removed from the 4s subshell before the D subshell?

Yes, that is correct.

Yes, that is correct.
By the way: why?

(serious question, not rhetorical)

By the way: why?

(serious question, not rhetorical)

*

kaplan instructor today confused the hell out of me and didnt explain it well when i asked him about it.
He said the reason P can have five bonds is because it has a d orbital, so i asked why dont we include the d orbital in the electron configuration, he said he wasnt sure.
Also van der walls equation, so we do have to memorize that? or is it more conceptual.
Oh and why doesnt low pressure cause a gas to deviate away from ideal gas law?

kaplan instructor today confused the hell out of me and didnt explain it well when i asked him about it.

1. He said the reason P can have five bonds is because it has a d orbital, so i asked why dont we include the d orbital in the electron configuration, he said he wasnt sure.

2. Also van der walls equation, so we do have to memorize that? or is it more conceptual.

3. Oh and why doesnt low pressure cause a gas to deviate away from ideal gas law?

1. I think that you are confusing hybridization of atoms that are bonding to other atoms with electron configurations for lone atoms. All atoms have d-orbitals, but elements that are in row 3 or greater of the periodic table have low-lying d-orbitals that are available for bonding. A phosphorus central atom with five bonds WOULD have one of its 3d orbitals included in its hybridization. In other words, when it has five bonds and trigonal bipyramidal geometry, its hybridization is sp3d. That d comes from one of the 3d orbitals in the phosphorus atom. So there are five sp3d orbitals, used to make the five bonds to the phosphorus. Keep in mind that elements in rows 1 and 2 will NEVER exceed their octets, because aren't any 1d or 2d orbitals to make sp3d or sp3d2 hybrid orbitals. So nitrogen, right above phosphorus, can never have five bonds.

This is different than the scenario for writing the electron configuration of a single atom. When you write out the electron configuration of a phosphorus atom, it won't be bonded to anything. So in that case, the 3d orbitals are empty and do not need to be written. They only come into play if sp3d hybridization is occurring.

2. No, there is no need to memorize the equation. Questions will be more concept focused.

3. Low pressure does not cause a gas to deviate from the ideal gas law since the increased spacing between molecules leads to decreased levels of interaction. Remember, the ideal gas law presumes non-polar gas molecules that do not interact via hydrogen bonding and dispersion forces. At reduced pressure, these assumptions are followed by the individual gas molecules to a greater degree than at high pressure where the chances of gas molecules interacting with each other are better.

Hey everyone,

I will help answer questions as much as I can. I have a Ph.D. in physical chemistry (just defended my thesis today!!). I took the mcat last august (9V, 14P, 13B). Don't ask me any verbal strategies, that's my weak point! :) I'm pretty good at physical sciences, though. I will be an M1 at Wash U next fall.

Sparky

What exactly is a bleach? What does "to bleach" mean?

What exactly is a bleach? What does "to bleach" mean?

lol, this will not be on the MCAT. :p

Household bleach like Clorox is an aqueous solution of sodium hypochlorite (NaClO). It's an oxidizing agent, which means that it accepts electrons from another species. But in your laundry, what we care about is that it is a whitening agent. Technically you could call peroxide whiteners like Oxyclean "bleaches" also. They are oxidizing agents as well.

lol, this will not be on the MCAT. :p

Household bleach like Clorox is an aqueous solution of sodium hypochlorite (NaClO). It's an oxidizing agent, which means that it accepts electrons from another species. But in your laundry, what we care about is that it is a whitening agent. Technically you could call peroxide whiteners like Oxyclean "bleaches" also. They are oxidizing agents as well.
How do they whiten? Not by oxidizing? Does bleaching mean oxidizing? Does that mean that all Lewis bases are bleaches?

Do you want me to go play in my own sandbox now?

How do they whiten? Not by oxidizing? Does bleaching mean oxidizing? Does that mean that all Lewis bases are bleaches?

Do you want me to go play in my own sandbox now?
The wavelength of light absorbed by a molecule is related to the length of the orbital in the molecule. Small orbitals, like those found in water or carbon dioxide or such, do not absorb light in the visible range of the spectrum, and they appear colorless to us. The sorts of things we see as colored are generally either metals (like the iron in blood that gives it a red color) or they're conjugated orbitals (like those in beta carotein that give it an orange color). Chlorophyl is another sort of molecule that has a great deal of conjugation in order to give the extended molecular structure that better absorbs light.

Strong oxidizers like sodium chlorite, sodium hypochlorite, and chlorine steal electrons and ruin these sorts of conjugated systems. This works both in making the molecule no longer absorb light, as well as in breaking up large molecules (like lipids) and turning them into water soluble components that you can wash away.

Bleaching can also be done by high energy light, in a process called photobleaching. If the energy of the photons employed in sufficient, than an electron that is excited by such a photon will have sufficient energy to bust up out of that orbital, and go for a solo project. This technique is used in molecular biological techniques like FRAP (fluorenscence recovery after photobleaching) where plasma membranes are made to incorporate fluorophores. The fluorophores are bleached with a high intensity light in a small region, making a bleached region on the membrane. The rate at which fluorescence returns to the bleached area is a measure of the fluidity of the membrane. In a highly fluid membrane, the fluorophores adjacent to the bleached area will diffuse into the region, rapidly destroying the bleach spot, while a relatively stable, non-fluid membrane would retain the bleach area for considerable time. This allows a measure of membrane fluidity in a specific region of membrane.

Not all lewis bases are necessarily bleaches--it all depends on the relative oxidizing power of an agent reletive to stbility the thing to be bleached.

The wavelength of light... to be bleached.
Bravo, and bless the new forum. Thank you.

...You can use the osmotic pressure equation to calculate what the osmotic pressure is for a given solution, such as 20 g of NaCl dissolved in 500 mL of water at STP. So if your cell is hypertonic to your solution, as in your example, then yes, the osmotic pressure is greater inside the cell versus outside. The hypertonic solution will always have a higher osmotic pressure because it has a higher molarity, and the temperature and gas constant are both constant. Don't forget to convert your temperature to Kelvins, not Celsius, and remember that salts dissociate into ions, so that 1 mole of NaCl is actually 2 moles (approximately) of ions.

Great point! I just want to emphasize that. It's one way to miss an easy question on the mcat, because you can bet the answer with 1 mole of ions will be an option.

i seem to consistently be missing questions about delta g, delta s, and delta h type stuff. i was wondering if you knew where i could go to get a better grasp on this type of information. it seems that i understand that spontaneous rxns are negative delta s...but the other stuff is a blur. This also has bio connotations....but i think i can kill 2 birds with one stone by putting it here. hope you can help.

this was meant for the chem section...sorry.... :eek:

Hi-

regarding your question, there are a couple of equations, but the one that helped me most was:

G = H - T(S)

where:
G= change in free energy (Gibbs free energy)
T=temperature
H= change in enthalpy
S= change in entropy
(there is an acronym for the equation: "Goose Hunters Take Shotguns" if that helps...)

spontaneous reactions occur when delta G is negative, not S... also, a reaction tends to be spontaneous when the products are more disordered than the reactants (a positive S), and when the reaction is exothermic (a negative H). i'm not big on physical sciences, so i'm sure some of the others here can elaborate on what i've said...

good luck!

Can you please give me some pointers on determing the excited state of an atom. Thank you

Can you please give me some pointers on determing the excited state of an atom. Thank you
The excited state of a system (such as an atom, molecule or nucleus) is any configuration of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum).


An example of this concept comes by considering the hydrogen atom.

The ground state of the hydrogen atom corresponds to having the atom's single electron in the lowest possible orbit (that is, the spherically symmetric "1s" state, which has the lowest possible quantum numbers). By giving the atom additional energy (for example, by the absorption of a photon of an appropriate energy), the electron is able to move into an excited state (one with one or more quantum numbers greater than the minimum possible). If the photon has too much energy, the electron will cease to be bound to the atom, and the atom will become ionised.

Once the electron is in its excited state, we deem the hydrogen atom to be in its excited state. The atom may return to a lower excited state, or the ground state, by emitting a photon with a characteristic energy. Emission of photons from atoms in various excited states leads to a spectrum showing a series of characteristic emission lines (including, in the case of the hydrogen atom, the Lyman series, and the Balmer series

Lyman series:
the series of transitions and resulting emission lines of the hydrogen atom as an electron goes from n ≥ 2 to n = 1

Balmer series:
The Balmer series is the series of transitions and resulting emission lines of the hydrogen atom as an electron goes from n ≥ 3 to n = 2

Hi!

I was reviewing this material yesterday and I'm still a little shaky on it.

I just know indicators are weak acids and when introduce to a soln it can determine its pH. I also know that there are different indicators that are specific to pH ranges... however I'm a little confuse on how the soln itself can affect the dissociation of the weak acid... (I hope I'm not confusing you).

For example... The indicator dissociates like this:

HA + H20 ---> H3O+ + A-
color 1 color 2

So if it's an acidic solution that the indicator is placed into will the H+ ions force the equation to the right therefore the indicator will turn into color 1???

Is it like Le Chauteliers?? Or am I just confusing the two???

Thank you so much for your input!!! GREAT THREAD BTW! :D

Hopeful023

I just know indicators are weak acids and when introduce to a soln it can determine its pH. I also know that there are different indicators that are specific to pH ranges... however I'm a little confuse on how the soln itself can affect the dissociation of the weak acid...

For example... The indicator dissociates like this:

HA + H20 ---> H3O+ + A-
color 1 color 2

So if it's an acidic solution that the indicator is placed into will the H+ ions force the equation to the right therefore the indicator will turn into color 1???

Is it like Le Chauteliers?? Or am I just confusing the two???

If I understand your question correctly, then you have the right idea. An indicator in solution more acidic than its pKa will be mostly protonated, and will be in its HA form. So it will take on that particular color. An indicator in a solution more basic than its pKa will be mostly deprotonated, and will turn the color of the A- species. Note that it is important to know the pKa of the indicator in order to know whether the indicator will be protonated or deprotonated at a given pH. That's why indicators are specific to given pH ranges as you mentioned.

THANK YOU SO MUCH! I was hoping that I got the concept correct :) and it feels nice that someone else can correct me if I was wrong.


If I understand your question correctly, then you have the right idea. An indicator in solution more acidic than its pKa will be mostly protonated, and will be in its HA form. So it will take on that particular color. An indicator in a solution more basic than its pKa will be mostly deprotonated, and will turn the color of the A- species. Note that it is important to know the pKa of the indicator in order to know whether the indicator will be protonated or deprotonated at a given pH. That's why indicators are specific to given pH ranges as you mentioned.

would you be so kind as to list the equations we need to remember?

I just wanted to say thank you. Your way of explaining concepts is amazingly clear and straightforward. I just took a summer exam on these very concepts; and while I did fine, I probably could have avoided a lot of page flipping by having read your posts beforehand.

You are doing an awesome job, and I (and I'm sure many other members of this site) really appreciate it

In regards to hybridization, is it reliable to know that the amount of electron domains will equal the hybridization state? As an example:

Nitrogen in NH3 has 4 domains, 1 free electron pair and 3 N-H bonds which give it an sp3 hybridization state. The central I in I3- would have an sp3d2 hybridization since it has two I-I bonds and 3 free pairs of electron, thus giving it 5 total domains.

Are there any exceptions to that rule?

Hi, I was reading over Raoult's Law. I understand the formula given is to determine the vapor pressure of a solute or solvent in a solution? But would the vapor pressure of the pure solvent or pure solute be given in order to figure out the difference in vapor pressure? And if a solute is added to a pure solvent will the vapor pressure ALWAYS decrease?

In regards to hybridization, is it reliable to know that the amount of electron domains will equal the hybridization state? As an example:

Nitrogen in NH3 has 4 domains, 1 free electron pair and 3 N-H bonds which give it an sp3 hybridization state. The central I in I3- would have an sp3d2 hybridization since it has two I-I bonds and 3 free pairs of electron, thus giving it 5 total domains.

Are there any exceptions to that rule?

Hi Belfagor,

This is a simple, powerful rule and you should use it unless you have good reason to consider other effects.

However, there are exceptions. For example, ammonia (NH_3) is hybridized sp3 and has a trigonal pyramid molecular geometry. Triphenylamine, which has 3 large phenyl groups (N(C6H5)_3) instead of 3 tiny hydrogens atoms, is hybridized sp2. The phenyl groups are large enough to force the molecule into a planar geometry.

(Triphenylamine is even more interesting, structurally. The bonds to the nitrogen atom are planar, but the phenyl groups twist about 40 degrees out of the plane, which causes the molecule to resemble a propeller!)


See you,
Sparky

I wish someone could explain how to do acid/base problems and they are hard and confusing. I wish there was an easier way to tackle them.

Hi, I was reading over Raoult's Law. I understand the formula given is to determine the vapor pressure of a solute or solvent in a solution? But would the vapor pressure of the pure solvent or pure solute be given in order to figure out the difference in vapor pressure? And if a solute is added to a pure solvent will the vapor pressure ALWAYS decrease?

1. The vapor pressure given is almost always that of the pure solvent in which the solute is dissolved.

2. Yes, you would usually be given the "null" vapor pressure (for pure solvent) and asked to solve for the new one with solute added. The pure solute will usually be a solid, and it will not have a vapor pressure unless it is a substance that sublimates, like solid iodine or carbon dioxide. But for MCAT-level gen chem, you should assume that all solid solutes are not going to sublimate and any liquid solutes are not volatile, unless you get a passage where they explain how to solve such problems.

2. Yes, the vapor pressure will decrease whenever a non-volatile solute is added to the solvent. Again, scenarios with volatile solutes are beyond the scope of the test. I will post a more thorough explanation of colligative properties in the gen chem explanations thread soon.

I wish someone could explain how to do acid/base problems and they are hard and confusing. I wish there was an easier way to tackle them.

I will add that to the list. Other future topics for this thread will be colligative properties, kinetics, and thermodynamics.

im confused about Ksp value determination and its relationship to Qsp,
is there some formula for Ksp values we should know like kaplan says for every compound with a formula MX3 than Ksp=27X^4

another question why is bond breakage endothermic, and bond forming exothermic,
dont you release energy in catabolic processes and need energy for anabolic processes? Why isnt enthalpy the same

Do u think we should know how to balance redox reactions? Its in my Kaplan book, but Examkrackers says its highly unlikely it will be asked on the MCAT? Thanks

Do u think we should know how to balance redox reactions? Its in my Kaplan book, but Examkrackers says its highly unlikely it will be asked on the MCAT? Thanks
I personally encountered questions requiring an understanding of redox reactions. Don't neglect it. It is pretty simple and the applications of the understanding can be very broad, helping with many related topics.

im confused about Ksp value determination and its relationship to Qsp,
is there some formula for Ksp values we should know like kaplan says for every compound with a formula MX3 than Ksp=27X^4

Don't blindly memorize formulas, because that is bound to get you into trouble if you are asked a question that is even the slightest bit different than one you've seen before. Instead, the best way to solve Ksp problems IMHO is to make up one of those little charts, with the starting concentrations, change in concentrations, and final concentrations for each species. (Technically, you can ignore the salt, since it's a pure solid and it won't appear in the equilibrium expression.) You might be asked to calculate Ksp using the molar solubility, or you might be given Ksp and asked to calculate Qsp to determine whether your salt will precipate or not. If Ksp<Qsp, the salt will precipitate (supersaturated solution), but if Ksp>Qsp (unsaturated solution), it will not.

Again, I would like to emphasize to everyone: to score well on these tests (MCAT, PCAT, DAT, or OAT), you must understand what you are doing; don't blindly memorize formulas.

another question why is bond breakage endothermic, and bond forming exothermic,
dont you release energy in catabolic processes and need energy for anabolic processes? Why isnt enthalpy the same

An endothermic process is one that requires an input of heat or energy to take place. The intuitive explanation is that in general, breaking anything will require an input of energy on my part, whether it's a bond, a pinata, or a glass window. Specifically looking at bonds, consider what you are trying to do when you break one. In a bond, you generally have two atoms with complete octets that are part of a neutral species, and when you separate them, you often end up having two charged species or two radicals, either of which is less stable (higher in energy) than what you started with. So bond breaking is endothermic because the products (separated atoms) are higher in energy than the bonded atoms were.

Bond forming is the reverse process of bond breaking, and it will be exothermic because now you are forming a product that is lower in energy (more stable) than the individual atoms are.

Comparing test-tube bond breaking and bond making to biological processes isn't actually analogous, because biological systems "cheat." They couple energetically favorable reactions, like the hydrolysis of ATP, with energetically unfavorable reactions. So yes, you do require an input of energy in anabolic processes, but it comes from the ATP hydrolysis that is driving those reactions, not the anabolic reactions themselves. Similarly, catabolic reactions do release some energy, but again, some energy must be inputted in order to get them going, and their products are new bonds (that's how the energy is stored). Consider glycolysis, for example, which requires two ATP to get going, and creates four total ATP per glucose, but only two net ATP per glucose when you subtract the two needed to start the reaction.

I personally encountered questions requiring an understanding of redox reactions. Don't neglect it. It is pretty simple and the applications of the understanding can be very broad, helping with many related topics.

I agree, and I'd add that you should never do any more work than you have to do to solve the question you are being asked. For example, if the question asks how many moles of electrons are being transferred, you don't have to go all the way through to getting the final equation. You can stop at the point where you multiply each half reaction by some integer so that they will each have the same number of electrons; the number of electrons is the answer to that question.

Okay, Could someone PLEASE!!!! help me out? I have like three text books open and I cannot seem to grasp the concept of Potentials and electrochemistry. Specifically I have an idea of how glavanic Cells work but not the relationships between K, Q, Delta G^o, Delta G, and T, and this funky letter Q that seems to mean Keq but I cannot seem to verify that as hard fact. If someone with any idea of what is going could just explain in a uncomplicated, fluid manner about how all those constants work together in electrochemisty I would be in eternal thankfulness. Thanks a bunch...for now I'll go back to pulling out my hair. :eek:

Okay, Could someone PLEASE!!!! help me out? I have like three text books open and I cannot seem to grasp the concept of Potentials and electrochemistry. Specifically I have an idea of how glavanic Cells work but not the relationships between K, Q, Delta G^o, Delta G, and T, and this funky letter Q that seems to mean Keq but I cannot seem to verify that as hard fact. If someone with any idea of what is going could just explain in a uncomplicated, fluid manner about how all those constants work together in electrochemisty I would be in eternal thankfulness. Thanks a bunch...for now I'll go back to pulling out my hair. :eek:

Keq is the equilibrium constant, and it is equal to the product of the concentrations of the products divided by the product of the concentrations of the reactants under equilibrium conditions. (In other words, multiply the equilibrium product concentrations together in the numerator, and the equilibrium reactant concentrations in the denominator. And of course you must raise each concentration up to its coefficient in the reaction equation.)

But what happens when you're not at equilibrium? You can still calculate the ratio of product concentrations to reactant concentrations, and that is what Q is. Q is calculated in exactly the same way as Keq is, but you use Q rather than Keq whenever you are not under equilibrium conditions. Q measures how far you are from equilibrium. If Q is greater than Keq, that means there are too many products and the reaction will go backward. On the other hand, if Q is less than Keq, then you don't have enough products, and the reaction will go forward, forming more products. This concept is frequently tested for salts by questions asking you to compare the ion product, Q, to the salt solubility equilibrium constant, Ksp.

Delta G is the free energy, which measures spontaneity of a reaction. You can use delta G any time, whether or not you are under standard conditions. (Standard conditions are 1 atm of pressure, 298 K, and 1M concentrations of each compound in your mix.) Delta G0 is specifically the free energy under standard conditions. So it is a special value of delta G in general.

Both Keq and delta G are temperature-dependent. Keep in mind that a Keq value is only valid at a given temperature. If you raise the temperature, generally the Keq will increase (favor the products more) because most familiar reactions are endothermic. Temperature will also affect the spontaneity of reactions, because delta G depends on both Keq and on absolute temperature. If T increases, it tends to make delta G more negative, and again, favors formation of more products.

Q,

Someone posted in the main MCAT forum about a discrepancy between EK and kaplan in what they say about volume of real gases vs. ideal gases. If you could so graciously clarify this issue, it would be appreciated by the original poster and any other people that may come across this issue.

But this is what the question was as quoted from the other page:

Hey Guys-quick question:

If you look in the EK Chemistry book on page 27, about 2/3 of the way down it shows Vreal> Videal, and then in the Kaplan PS book, page 133, 3rd paragraph, it says that that real gases have less volume than ideal gases. I agree with Kaplan since in a real gas, pressure is greater, causing molecules to be pushed closer together. Am I missing something here? Or is it a typo? Any help would be greatly appreciated. Thanks!

Someone posted in the main MCAT forum about a discrepancy between EK and kaplan in what they say about volume of real gases vs. ideal gases.

[docmd2010]:If you look in the EK Chemistry book on page 27, about 2/3 of the way down it shows Vreal> Videal, and then in the Kaplan PS book, page 133, 3rd paragraph, it says that that real gases have less volume than ideal gases. I agree with Kaplan since in a real gas, pressure is greater, causing molecules to be pushed closer together. Am I missing something here? Or is it a typo? Any help would be greatly appreciated. Thanks!

You have misinterpreted the Kaplan book here. It is correct that at high pressure, a real gas occupies more volume than the volume predicted for an ideal gas. The Kaplan book DOES say this; look one sentence further in that same paragraph. ;) You stopped on the sentence about moderate pressures, where intermolecular interactions predominate, and cause the volume to be smaller than predicted for the ideal gas.

Basically, there are two factors that cause real gas behavior to deviate from ideal gas behavior. One is the attraction that exists among gas particles, which becomes significant at medium pressure; and the other is the volume of the gas particles themselves, which is significant at high pressure. The ideal gas law assumes that these two factors are insignificant, which is generally true at low pressures. As you start raising the pressure to a medium level, the gas particles begin to attract one another. This causes them to "stick" together and contracts the volume of the gas overall below what would be expected if no attraction were occurring. As you continue to raise the pressure, the volumes of the individual gas particles begin to occupy a significant portion of the volume of the container. This causes the volume of the gas to be larger than expected, because the ideal gas law does not take into account the volumes of the gas molecules. (According to the ideal gas law, the molecules have no volume, as well as no intermolecular interactions.)

Hope this helps.

You have misinterpreted the Kaplan book here. It is correct that at high pressure, a real gas occupies more volume than the volume predicted for an ideal gas. The Kaplan book DOES say this; look one sentence further in that same paragraph. ;) You stopped on the sentence about moderate pressures, where intermolecular interactions predominate, and cause the volume to be smaller than predicted for the ideal gas.

Basically, there are two factors that cause real gas behavior to deviate from ideal gas behavior. One is the attraction that exists among gas particles, which becomes significant at medium pressure; and the other is the volume of the gas particles themselves, which is significant at high pressure. The ideal gas law assumes that these two factors are insignificant, which is generally true at low pressures. As you start raising the pressure to a medium level, the gas particles begin to attract one another. This causes them to "stick" together and contracts the volume of the gas overall below what would be expected if no attraction were occurring. As you continue to raise the pressure, the volumes of the individual gas particles begin to occupy a significant portion of the volume of the container. This causes the volume of the gas to be larger than expected, because the ideal gas law does not take into account the volumes of the gas molecules. (According to the ideal gas law, the molecules have no volume, as well as no intermolecular interactions.)

Hope this helps.



Thanks a lot Q. Crystal clear explanation. :D

In a graph comparing internuclear distance (H-H) vs it's potential energy (electrostatic) do we need to know anything more than where to find the bond length and the bond dissociation energy?

What does the area above the curve indicate?

My trouble w/ the graph right now is the graph going from left to right, can somebody explain to me in layman terms why the potential energy coming from the left appears to be very high? 1st of all am I correct in thinking that it is b/c the 2 H's are very close together? 2ndly am I correct in assuming that as the curve approaches zero it never reaches zero no matter how far they are apart b/c one H will always 'feel' the other? Is that what the curve is telling me???

Thanks

Howdy,

I was just reviewing over Equilibrium and Acids and Bases. It seems to me that there are a truckload of formulas to remember.

Ones for Ksp, Keq, Kb, Ka, pKa, pKb, how they relate to eachother, pH, pOH, how to find concentration of something in a buffer solution, how to find pH with strong acids/bases, and on and on. Can someone post a concise explanation of Equilibrium and acid/base stuff? I know it seems like a lot to ask... but... yeah :rolleyes:

How detailed/tedious is the electrochemisty and the acid base chemistry on the MCAT? I took Analytical Chem over a year ago but don't remember much and am worried about how detailed it can get. I am confident with what is in the review books as it is really basic compared to what we learned in class.

I guess my question would be: Is it enough to know everything in the TPR Physical Sciences review book or should I crack open my anal. chem book and study the related topics in detail?

I guess my question would be: Is it enough to know everything in the TPR Physical Sciences review book or should I crack open my anal. chem book and study the related topics in detail?
The TPR books are plenty. I strongly suspect that's true of Kaplan's, EK's, and so on, also.

I have not seen a biology or chemistry book , other than TPR's Science Reviews, since before many of you were born. I scored 14, twice, in the PS section, and 12 in BS without any O-chem (even ours).

Need some help on figuring out what is more soluble as I can’t seem to find it explicitly stated.

PbSO4 (Ksp=1.8E-8) vs. CaSO4 (Ksp=2.4E-5)
Ag2SO4 (Ksp=1.7E-5) vs. CaF2 (Ksp=3.9E-11) ion ratio matters? 2:1 and 1:2
PbSO4 (Ksp=1.8E-8) vs CaF2 (Ksp=3.9E-11) Notice how the ion ratio is different
BaSO4 (molar solubility=1.1E-5 mol/L) vs. Ag2CrO4 (molar solubility=1.3E-4mol/L)

Also could someone please explain the intuition behind using with the Ksp formula? This seems like it should be rather simple but I put about 30minutes into it and have not made progress. Thanks!

Need some help on figuring out what is more soluble as I can’t seem to find it explicitly stated.

PbSO4 (Ksp=1.8E-8) vs. CaSO4 (Ksp=2.4E-5)
Ag2SO4 (Ksp=1.7E-5) vs. CaF2 (Ksp=3.9E-11) ion ratio matters? 2:1 and 1:2
PbSO4 (Ksp=1.8E-8) vs CaF2 (Ksp=3.9E-11) Notice how the ion ratio is different
BaSO4 (molar solubility=1.1E-5 mol/L) vs. Ag2CrO4 (molar solubility=1.3E-4mol/L)

Also could someone please explain the intuition behind using with the Ksp formula? This seems like it should be rather simple but I put about 30minutes into it and have not made progress. Thanks!

Ksp is an equilibrium constant for the dissociation of a sparingly soluble salt. You set it up as you do any equilibrium constant by multiplying the concentrations of the products in the numerator, and dividing that by the concentrations of the reactants. However, since the reactants in this case are pure liquids and pure solids (salt and water), they do not appear in the equilibrium expression. (We assume that the concentrations of pure solids and pure liquids don't change much.) Thus, only the ion concentrations from the product side of the equation, raised to their coefficients, will appear in the equilibrium expression.

Now to your specific question: determining which compound has a greater solubility depends on the Ksp, but also on the number of ions as you said.
To compare two salts that form the same number of ions, you can simply inspect their Ksp values. But if they yield different numbers of ions, the situation is a little more complicated. Start by figuring out the molar solubility in mol/L of each salt. Then, if you convert your concentrations to g/L (do this by multiplying the molar solubility of the salt by its molar mass), you will get the net solubility.

Ex. CaF2: Ksp = 3.9 x 10^-11
equation: CaF2 -> Ca2+ + 2 F-
This leads to x as the molar solubility of Ca2+ and 2x as the molar solubility of F-
Ksp expression: Ksp = [Ca2+][F-]^2 = x (2x)^2 = 4x^3
x = molar solubility = (2.32 x 10^-4 mol/L)(78 g/mol) = 0.018 g/L

Repeat this process for the other salt with which you want to compare it. The one with the greater solubility in g/L is more soluble, even if it has a smaller Ksp. (This happens frequently if you are comparing salts with different numbers of ions.)

can someone explain to me real gases vs ideal?

on EK, it said V real > V ideal. and that P real < P ideal. when i took TPR, they said that "the ideal world is greater than the real world"...which means they contradict EK for Volume. why?

also, on EK, why did they have a picture of two boxes, one big and one small. the smaller ones was packed with molecules. this seemed also contradictory, because it makes you think real gases have high pressure, and low volume.

:confused:

can someone explain to me real gases vs ideal?

on EK, it said V real > V ideal. and that P real < P ideal. when i took TPR, they said that "the ideal world is greater than the real world"...which means they contradict EK for Volume. why?

also, on EK, why did they have a picture of two boxes, one big and one small. the smaller ones was packed with molecules. this seemed also contradictory, because it makes you think real gases have high pressure, and low volume.

:confused:

This question has been answered on the previous page of this thread (post #60): http://forums.studentdoctor.net/showthread.php?p=2755702#post2755702

I don't have access to EK or TPR books, so I can't answer any specific questions about them, but hopefully this general explanation about real versus ideal gases will help you.

I'm doing the EK 1001 Gen Chem questions, and have checked the EK website for typos on this, and haven't found any.

Number 51 on page 4 says:

Which of the following is ordered correctly in terms of atomic radius, from smallest to largest?

A. Al_3+, Al, S, S_2- (these numbers are supposed to be charges)
B. Al_3+, S, Al, S_2-
C. S, Al_3+, S_2-, Al
D. S, S_2-, Al_3+, Al

I picked A, but the back of the book says:

"B is correct. Positive ions are much smaller than their neutral counterparts; negative ions are much larger. Since the size of neutral atoms decreases as you move from left to right across the periodic table, neutral Al is bigger than neutral sulfur, and the correct answer is B."

I thought elements increased in size going left to right? Have I missed one of the most fundamental concepts in Chemistry?

Yes. See the two posts on periodic trends here: http://forums.studentdoctor.net/showthread.php?t=210973

From Kaplan's Physical Sciences Review Notes, pg 30.

The following electron configuration is given as belonging to group VIIB elements:

1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 4d^5 5s^2

I'm a bit confused, having figured that it would belong to group VB elements.

My reasoning: Shouldn't the 4d subshell fill completely before the 5s; if not, how are the electrons that would normally fill the valence shells of Rb and Sr accounted for since those are where the 5s would be attributed....?

wuts the diff bt extensive and intensive property? :confused:

wuts the diff bt extensive and intensive property? :confused:
Intensive properties are ones that do not depend on the sample size, such as temperature, pressure, pH, etc. Extensive properties do depend on the sample size, such as mass, thermal energy, volume, etc.

QofQuimica -
I have been trying the annotation strategy. Developing it like you suggested. Were you identifying what was in each paragraph, making a little note on the side to both help you understand quickly what was going on in each paragraph and where to find it if a question was asked (For G-chem)? Did you do the same thing for Physics? Sometimes physics passages have no diagrams, nor tables, so that is why I am considering using annotation for all passages to tap into the higher learning processes those neuropsychologists boast about. Thanks for your help!!

QofQuimica -
I have been trying the annotation strategy. Developing it like you suggested. Were you identifying what was in each paragraph, making a little note on the side to both help you understand quickly what was going on in each paragraph and where to find it if a question was asked (For G-chem)? Did you do the same thing for Physics? Sometimes physics passages have no diagrams, nor tables, so that is why I am considering using annotation for all passages to tap into the higher learning processes those neuropsychologists boast about. Thanks for your help!!

Hi Frank,

Yes, I do annotate every passage. For the science passages, my annotations are especially short. For example, if you have a passage with a paragraph describing how a rocket gets launched, there will probably be lots of numbers given about the speed of the rocket, the air resistance, whatever. Don't worry about the numbers. Instead, just note "rocket launch" in the margin, and that way if you get asked to calculate something about the rocket's launch, you know to go back to that paragraph to find the needed data. I want to emphasize again that the point of annotating is to ORGANIZE yourself so that you can find the data you need quickly, not to actually LEARN the data, much of which you will never be asked about in a question.

From Kaplan's Physical Sciences Review Notes, pg 30.

The following electron configuration is given as belonging to group VIIB elements:

1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^10 4p^6 4d^5 5s^2

I'm a bit confused, having figured that it would belong to group VB elements.

My reasoning: Shouldn't the 4d subshell fill completely before the 5s; if not, how are the electrons that would normally fill the valence shells of Rb and Sr accounted for since those are where the 5s would be attributed....?

I think what is confusing you is that you are thinking of the 4d electrons as part of the core. But they are actually valence electrons for the row 5 transition metals, as are the 5s electrons. So Zr, for example, has four valence electrons (two from 5s, two from 4d), not just two. You may also be confusing the 4d block with the 3d block. Keep in mind that the 3d block is in row 4 (between the 4s and 4p blocks) but the 4d block is in row 5, AFTER the 5s block.

Based on the Aufbau principle, Rb and Sr would both have empty 4d orbitals because the 5s orbital fills before the degenerate 4d orbitals do. The 4d and 5s subshells are very close to each other in energy. It actually turns out that the 5s subshell is slightly lower in energy when both are unoccupied, so the 5s will fill first before the 4d. However, when both subshells have electrons in them and you are removing electrons to form cations, the 5s is slightly higher in energy, and will tend to have its electrons removed first.

Hope that helps.

Intensive properties are ones that do not depend on the sample size, such as temperature, pressure, pH, etc. Extensive properties do depend on the sample size, such as mass, thermal energy, volume, etc.


how come when you dividie extensive with extensive, you get intensive?

how come pressure doesnt depend on sample size? if u have a lot of gas molecules in a jar, wouldnt the pressure be great?

I think what is confusing you is that you are thinking of the 4d electrons as part of the core. But they are actually valence electrons for the row 5 transition metals, as are the 5s electrons. So Zr, for example, has four valence electrons (two from 5s, two from 4d), not just two. You may also be confusing the 4d block with the 3d block. Keep in mind that the 3d block is in row 4 (between the 4s and 4p blocks) but the 4d block is in row 5, AFTER the 5s block.

Based on the Aufbau principle, Rb and Sr would both have empty 4d orbitals because the 5s orbital fills before the degenerate 4d orbitals do. The 4d and 5s subshells are very close to each other in energy. It actually turns out that the 5s subshell is slightly lower in energy when both are unoccupied, so the 5s will fill first before the 4d. However, when both subshells have electrons in them and you are removing electrons to form cations, the 5s is slightly higher in energy, and will tend to have its electrons removed first.

Hope that helps.

I really appreciate you getting back to me.
It still isn't quite clear. For your example with Zr, if it has four valence electrons would it be 5s^2 4d^2? If so, then adding 3 more electrons to the 4d subshell would put the original configuration in the period of VB, specifically Tc (not the Rh that Kaplan cites).

To put it another way, if only the 5s subshell filled (5s^2), and the 4d^5 were taken off of that original configuration, we'd have Sr right? So adding 5 more valence electrons to the configuration (to 4d^5) should put us at Tc....??

Is there any time on the MCAT where pKa will be of use besides for titrations?

how come when you dividie extensive with extensive, you get intensive?

This process is called "normalization." You already know that multiplying and dividing by the same number at the same time is permissible because it is equivalent to multiplying or dividing by one. (Ex. I can multiply by 2/2 or 13/13 without changing the net value.) Similarly, if you have an extensive property in both the numerator and the denominator of a ratio, the mass of the sample gets divided out (m/m), and the resulting property is mass-independent, or intensive. A good example of that is density, which is intensive, and which is also equal to (mass/volume), both of which are extensive properties.

how come pressure doesnt depend on sample size? if u have a lot of gas molecules in a jar, wouldnt the pressure be great?

Pressure is equal to (force/area), both of which are extensive properties. Thus, pressure is intensive as explained above. It's true that adding more gas molecules to a container would raise the force exerted by the molecules on that container. However, it would also increase the volume of the container, which increases the container surface area upon which the molecules are exerting their forces. Thus, the net ratio of (force/area) remains constant, assuming that your jar is able to expand its volume when the force on it increases due to placing more molecules inside of it.

I really appreciate you getting back to me.
It still isn't quite clear. For your example with Zr, if it has four valence electrons would it be 5s24d2? If so, then adding 3 more electrons to the 4d subshell would put the original configuration in the period of VB, specifically Tc (not the Rh that Kaplan cites).

To put it another way, if only the 5s subshell filled (5s2), and the 4d5 were taken off of that original configuration, we'd have Sr right? So adding 5 more valence electrons to the configuration (to 4d5) should put us at Tc....??

Ok, now I understand why you are confused: you're misnumbering the d-block groups. The d-block actually starts with Group 3B, which includes Sc, Y, and La. Group 1B is all the way over on the right side of the d-block and includes Cu, Ag, and Au. There are actually three groups of elements that together are all called "7B"; these are Mn and its family, Fe and its family, and Co and its family. The 7B elements in period five would include Tc, Ru and Rh. Group 5B is comprised of V, Nb, and Ta.

Is there any time on the MCAT where pKa will be of use besides for titrations?

pKa is the equilibrium constant for an acid dissociating into its conjugate base and H+. Thus, you could get asked to calculate pKa given the molar solubility of an acid, or you could get asked to calculate the molar solubility given the pKa. pKa values are also useful to gauge relative strengths of acids and bases: stronger acids have lower pKas, while stronger bases have higher pKas.

Could someone please explain to me Ksp again? I know it has to do with solubility but there's always problems where I get mixed up when they give you the Ksp and ask about concentration and ions and I always get lost. And isn't there something to do with exceeding the value which causes formation in a solvent?

would you do this by finding the Ka and then finding -log pKa?

pKa is the equilibrium constant for an acid dissociating into its conjugate base and H+. Thus, you could get asked to calculate pKa given the molar solubility of an acid,

Could someone please explain to me Ksp again? I know it has to do with solubility but there's always problems where I get mixed up when they give you the Ksp and ask about concentration and ions and I always get lost. And isn't there something to do with exceeding the value which causes formation in a solvent?

See if this helps answer your question:
http://forums.studentdoctor.net/showpost.php?p=2773625&postcount=52
and
http://forums.studentdoctor.net/showpost.php?p=2773634&postcount=61

would you do this by finding the Ka and then finding -log pKa?

Yes. Ka is the equilibrium constant for dissociation of an acid, and pKa is the negative log of Ka. Anytime you see "p" in front of anything, that means that you should take the -log of it. In general for an acid:

HA <-> (H+) + (A-)

Ka = ([H+][A-])/(HA)

pKa = -log Ka

Hello Everyone,

I was looking at the AAMC list of General Chemistry topics and there are a few things on the list that i was hoping you guys could clarify for me.

They are as follows:

A. THE IONIC BOND (ELECTROSTATIC FORCES BETWEEN IONS)
1. E = kQ1Q2/d
2. E = lattice energy
3. Force attraction = R(n+e)(n-e)/d2

as well as

B. SOLUBILITY
3. Common-ion effect; its use in laboratory separations
a. complex ion formation ]
b. complex ions and solubility
c. solubility and pH

I would really appreciate it if someone could explain these to me.

Thanks so much in advance. :) :) :)

Hello Everyone,

I was looking at the AAMC list of General Chemistry topics and there are a few things on the list that i was hoping you guys could clarify for me.

They are as follows:

A. THE IONIC BOND (ELECTROSTATIC FORCES BETWEEN IONS)
1. E = kQ1Q2/d
2. E = lattice energy
3. Force attraction = R(n+e)(n-e)/d2

as well as

B. SOLUBILITY
3. Common-ion effect; its use in laboratory separations
a. complex ion formation ]
b. complex ions and solubility
c. solubility and pH

I would really appreciate it if someone could explain these to me.

Thanks so much in advance. :) :) :)

Hi bnoosha, and welcome to our humble forum. Please don't post your threads in multiple places. You can find answers to some of your questions here:

ionic bonds: (http://forums.studentdoctor.net/showpost.php?p=2756252&postcount=4)

Ksp post 1 (http://forums.studentdoctor.net/showpost.php?p=2773625&postcount=52)

Ksp post 2 (http://forums.studentdoctor.net/showpost.php?p=2773634&postcount=61)

I'll add the other topics to my gen chem list. Thanks for your suggestions. :thumbup:

can you please explain the following for me? :/

1) can you explain the difference bt Cv and Cp?

2) for pv = nrt, why do they keep talking about holding things constant?? like if pressure were constant, volume were constant.. :(

3)what does it mean when a soln is saturated?

4) if you are doing electron configuration for an atom, but the atom has a + charge,how do u know where to take the e- from?

5) why do pure substances have a mol fraction of 1?

thanks!

A solution of sodium chloride in water has a vapor pressure of 19.9 torr at 25°C. What is the mole fraction of NaCl in this solution? The vapor pressure of pure water is 23.8 torr at 25°C

When calculating the Molarity of a salt, isnt it pretty straight forward. mol/L. However, dissociation of salts in (aq) affects the Molarity in the term of the Osmotic pressure equation(and all other colligative properties for that matter). This concept does not make sense to me. If Molarity changes for the OP equation, why does it not change when calculating the Molarity of a salt in water?

1) can you explain the difference bt Cv and Cp?

If you mean c, the specific heat, then Cv is the specific heat for a constant volume process, and Cp is the specific heat for a constant pressure process. You do not need to learn about these concepts for the MCAT. For the MCAT, you should know how to calculate the amount of energy needed to raise a given amount of material by some number of degrees, using the formula Q = mc(deltaT).

2) for pv = nrt, why do they keep talking about holding things constant?? like if pressure were constant, volume were constant.. :(

If you hold some variables constant, you can "cheat" on the math and avoid the need to use the whole ideal gas formula. For example, if I know that I'm changing volume and pressure at constant temperature, I can use Boyle's Law instead of the ideal gas law: P1V1=P2V2. This is helpful b/c I don't have to put in values for R, n, or T. Remember that on the MCAT you have no calculator and you are being timed. So you should take shortcuts with the math whenever possible.

3)what does it mean when a soln is saturated?

Salts can be dissolved into water or some other solvent up to a certain point called the saturation point. Beyond this concentration, any further salt added to the solution will simply sink to the bottom as a precipitate. A solution is called "saturated" when no more salt can be dissolved in it.

4) if you are doing electron configuration for an atom, but the atom has a + charge,how do u know where to take the e- from?

Take it from the valence shell (outermost subshell). The core electrons should never be removed.

5) why do pure substances have a mol fraction of 1?


Mole fraction is defined as the ratio of the number of moles of a given substance divided by the total number of moles of all substances present. If you only have a single substance present, then the numerator and denominator will be equal, giving you a mole fraction of one.

A solution of sodium chloride in water has a vapor pressure of 19.9 torr at 25°C. What is the mole fraction of NaCl in this solution? The vapor pressure of pure water is 23.8 torr at 25°C

Let P = pressure and X = mole fraction.

P(NaCl sol'n) = [X(NaCl sol'n)][total P]
X(NaCl sol'n) = [P(NaCl sol'n)]/[total P] = 19.9/23.8

which is about 5/6 (reduce from 20/24), which is about 0.87. But that's not the answer you're looking for, b/c that's the X of the water. (I know this b/c the water is the substance that's having the vapor pressure, not the salt.) So, assuming that only salt and water are present in your solution, the mole fraction of salt must be 1-0.87, which is about 0.13.

When calculating the Molarity of a salt, isnt it pretty straight forward. mol/L. However, dissociation of salts in (aq) affects the Molarity in the term of the Osmotic pressure equation(and all other colligative properties for that matter). This concept does not make sense to me. If Molarity changes for the OP equation, why does it not change when calculating the Molarity of a salt in water?

It's because colligative properties depend on the total number of moles of all particles, not just the number of moles of a single type of particle. Dissociation of the salt affects the molarity used to calculate colligative properties because you have changed the number of moles of particles. Remember, for colligative properties, we don't care what the identity of the particles are. In contrast, molarity of the salt does depend on particle identity. So if I have one mole of NaCl and it dissociates to 1 mole of Cl- and 1 mole of Na+, then I now have two moles of particles. So if I tell you to calculate the number of moles of NaCl (1 mole in this case), that is different than if I ask you to calculate the number of moles of particles (2 moles in this case). You would use the latter number to calculate the molarity for any colligative property.

Thanks so much for all the time you take out for us Q! I arrived at the same conclusion as you did although I used Raoult's law - similar effect in this case. Unfortunately the answer is incorrect. Here is the catch - NaCl disassociates into Na+ ions and Cl- ions and it throws in twice the punch per mole NaCl. So,

X (NaCl) = 0.5 * [1 - X(H2O)] = 0.5 * [1 - 19.9/23.8]

Which would give about 0.08 or so. Another way to look at it:

2 X(NaCl) = 1 - X(H2O). I'm using the theoretical van't Hoff factor here (i=2). In reality, the multiplication factor would be slightly less.

Let P = pressure and X = mole fraction.

P(NaCl sol'n) = [X(NaCl sol'n)][total P]
X(NaCl sol'n) = [P(NaCl sol'n)]/[total P] = 19.9/23.8

which is about 5/6 (reduce from 20/24), which is about 0.87. But that's not the answer you're looking for, b/c that's the X of the water. (I know this b/c the water is the substance that's having the vapor pressure, not the salt.) So, assuming that only salt and water are present in your solution, the mole fraction of salt must be 1-0.87, which is about 0.13.

Unfortunately the answer is incorrect. Here is the catch - NaCl disassociates into Na+ ions and Cl- ions and it throws in twice the punch per mole NaCl.

You're right. You'd have about 0.13 mole fraction of ions (by my calculation), but that isn't what the question asked for. (For the MCAT, you can assume that 1 mole of NaCl dissociates completely into 2 mol ions.)

Actually, this is a perfect example of why it is so important to always make sure that you understand exactly what question you are being asked, and to answer that exact question. It is very possible on all of these standardized tests to come up with a "correct" answer that is wrong in that context, because it is not answering the question that you are actually being asked.

It seems like so many questions on the MCAT can be solved through using conversion charts, yet I cant seem to get it right. They always hand you so many values of mass,N,m, or M, coupled with the fact that you have a reaction to look at, and I never know where to start. I guess my questions is, how do you generally tackle these problems?

For example, this questions seemed as though a conversion chart could solve it:
What is the molality of a stock solution that is 10 percent SDS by mass?(MW of SDS=288). Wouldnt you just try to make a chart that ends with mol SDS/kg soln? If so, where do you start?

* If you cant solve this through a conversion chart, just answer the two questions seperately please.

So I have two questions. I was doing EK 1001 passages for G-chem and I ran into a question that completely confused me and I was wondering if someone could explain it to me.

Cations in the salt bridge of a galvanic cell move:
a. From the positively charged cathode side to the negatively charged anode side.
b. From the negatively charged anode side to the positively charged cathode side.
c. From the positively charged anode side to the negatively charged cathode side.
d. From the negatively charged cathode side to the positively charged anode.

I said the answer is b but the book said the answer is d. I'm not sure why the answer is d. The electrons move from negative to positive but I don't know if that's the same for cations (positively charged anions). Please explain the rationale behind this question. Thanks

The other question is regarding oxidation numbers..
when you're doing oxidation number for example, MnPO4. What would the oxidation state of Manganeese be? How would you determine if there basically two unknowns. You only know that Oxygen has a -2. What about phosphorus and Manganese?

Please let me know..

It seems like so many questions on the MCAT can be solved through using conversion charts, yet I cant seem to get it right. They always hand you so many values of mass,N,m, or M, coupled with the fact that you have a reaction to look at, and I never know where to start. I guess my questions is, how do you generally tackle these problems?

For example, this questions seemed as though a conversion chart could solve it:
What is the molality of a stock solution that is 10 percent SDS by mass?(MW of SDS=288). Wouldnt you just try to make a chart that ends with mol SDS/kg soln? If so, where do you start?

* If you cant solve this through a conversion chart, just answer the two questions seperately please.

I'm not sure what you mean by "conversion charts." Are you talking about using dimensional analysis to solve stoichiometry problems? If so, here are the general steps for doing it:

1. Write out and balance the reaction equation. This is always a good place to start, because you need to understand what the problem is before you can hope to solve it. This step is not really applicable to your example, because it isn't a reaction.
2. Write down the information you've been given, and also what you are looking for. Again, this is part of understanding what the question is asking you to do. In your example, we have 10% SDS by mass, SDS has MW of 288 g/mol, and we are looking for its molality.
3. Start with what you know, and work toward what you would like to know. If I know I have a 10% SDS solution, then that means 1000g of that solution would contain 100g of SDS and 900g water. Molality is the mol of solute (SDS in this case) divided by the kg of solvent (water in this case). First I will solve for moles of solute:

100g SDS x (1 mol SDS)/(288g SDS) is about equal to 1/3 (it's slightly larger than 1/3, actually; let's say it comes out to about 0.35 mol SDS.)

Now, I have to divide 0.35 moles by my 0.9 kg of water (careful here; don't leave your solvent mass in g!), which is again slightly more than 1/3. We'll say it is 0.37 m. And we're done. If you use a calculator, you'll see that we are very close (actual answer 0.385 m).

Now, you might wonder, how did I know to pick 1000g to start with? The answer is that I can pick any amount I want, and the result will come out the same. The key is to try to pick a number that is easy to work with, since we can't use a calculator. If I chose to use 300g of solution, I'd have 30g of SDS in 270g of water, and my molality would still be 0.385. If you're not convinced, try a few other numbers yourself; molality is an intensive property, so it won't matter how much solution you use.

Cations in the salt bridge of a galvanic cell move:
a. From the positively charged cathode side to the negatively charged anode side.
b. From the negatively charged anode side to the positively charged cathode side.
c. From the positively charged anode side to the negatively charged cathode side.
d. From the negatively charged cathode side to the positively charged anode.

I said the answer is b but the book said the answer is d. I'm not sure why the answer is d. The electrons move from negative to positive but I don't know if that's the same for cations (positively charged anions).

I think there is a typo in your book; check the EK website to see if anyone has reported an error for that question. The cathode is positive, not negative, for a galvanic cell, and the cathode is negative for electrolytic cells. In all types of cells, the electrons always move to the cathode, which is where reduction occurs. If you have a spontaneous cell (galvanic), then your cathode will be positive. This makes intuitive sense; the negatively charged electrons "want" to go to a positively charged electrode if they can. Cations in the salt bridge must move to balance the electrons so that you do not have a buildup of charge in the cathode. Thus, they will also move to the cathode, following the electrons. The salt bridge anions will go toward the anode to balance out the electrode cations left behind after the electrons leave.

The other question is regarding oxidation numbers..
when you're doing oxidation number for example, MnPO4. What would the oxidation state of Manganeese be? How would you determine if there basically two unknowns. You only know that Oxygen has a -2. What about phosphorus and Manganese?


You should memorize your common polyanions. PO4 anion has a charge of 3- on it. Therefore, Mn must be 3+ to balance this out. Phosphorus would have an oxidation number of +5 (the four O's total to -8).

I think there is a typo in your book; check the EK website to see if anyone has reported an error for that question. The cathode is positive, not negative, for a galvanic cell, and the cathode is negative for electrolytic cells. In all types of cells, the electrons always move to the cathode, which is where reduction occurs. If you have a spontaneous cell (galvanic), then your cathode will be positive. This makes intuitive sense; the negatively charged electrons "want" to go to a positively charged electrode if they can. Cations in the salt bridge must move to balance the electrons so that you do not have a buildup of charge in the cathode. Thus, they will also move to the cathode, following the electrons. The salt bridge anions will go toward the anode to balance out the electrode cations left behind after the electrons leave.



You should memorize your common polyanions. PO4 anion has a charge of 3- on it. Therefore, Mn must be 3+ to balance this out. Phosphorus would have an oxidation number of +5 (the four O's total to -8).


So the answer should be b right to the first question?

So the answer should be b right to the first question?

Based on the question as written, then yes, B seems to be the right answer. C and D can't be correct for a galvanic cell because the cathode should be positive, not negative.

Hi! I am having trouble with the concepts of vapor pressure. Is that the same as "pressure"? And it's relationship to boiling point. If you boil a pot of water at higher altitude, would it boil faster? Why is it that at higher altitude, the atmospheric pressure is lower?


How about a high-pressure cooker? Why does your food cook faster in this case?

Sorry, so many whys! I find this topic to be very counter-intuitive.

Thanks!

The density of a 3.54 M solution of NH4Cl in water is 1.0512 g/mL. What is the molality of the solution? (The molar mass of NH4Cl is 53.45 g/mol.)
Molality is defined as the number of moles per kilogram of solvent, and molarity is the number of moles per liter of solution. So a liter of the solution weighs:

1.0512 g/mL * 1000 mL/L * .001 kg/g * 1 L = 1.0512 kg

In that liter, there is 3.54 moles of NH4Cl, which weighs:

3.54 mol/L * 53.45 g/mol * 1 L * .001 kg/g = 0.189 kg

So the mass of the water in the liter is the difference of the total mass less the mass of the solute = 1.0512 kg - 0.189 kg = 0.862 kg

So the number of moles of solvent in a kilogram of water would be found by looking at the ratio 1 kg : 0.862 kg. Hence,

Molality =
3.54 mol/Lsolvent / (0.862 kgsolvent/L)) = 4.11 mol/kgsolvent

Hi! I am having trouble with the concepts of vapor pressure. Is that the same as "pressure"? And it's relationship to boiling point. If you boil a pot of water at higher altitude, would it boil faster? Why is it that at higher altitude, the atmospheric pressure is lower?


How about a high-pressure cooker? Why does your food cook faster in this case?

Sorry, so many whys! I find this topic to be very counter-intuitive.

Thanks!
Vapor pressure is the part of the total pressure that is accounted for by the vapor. Ideally, this means that if the total pressure is 12 atm, and the air is 25% vapor, then the vapor pressure is 25% of 12 atm = 3 atm. In a closed system, the amount of vapor in the gas phase will be set by the saturation vapor pressure. The central idea here is that of equilibrium.

Equilibrium is often thought of as the point where movement stops. In chemistry, a better way to think of it is that the rate in one direction and the rate in the other direction are equal and opposite, so there is no net movement.

In the case of the saturation vapor pressure, this means that for every molecule in the liquid that gets enough energy to break free of the liquid and enter the gas phase, there is a molecule already in the gas phase that strikes the surface of the liquid, and lacks the energy necessary to bounce back, hence remaining in the liquid. There is no net change in the ratio of molecules in the gas phase to molecules in the liquid phase.

For pressure in general, one must continue to think of this in terms of the single molecule colliding against others. If the pressure is high, then it will be harder (ie requires more energy) for a molecule to break free of the liquid and become a vapor, because the gas phase is crowded with other molecules. Under low pressure, there's more space, and hence it is easier (ie requires less energy) for a molecule to break free of the liquid and become a vapor.

When you boil water, the water will get hot enough to vaporize, but no hotter--because if it got hotter than needed to vapaorize, it would have already evaporated. So if the boiling point of water is 100 degrees, and you have boiling water, then you know that the water is 100 degrees--any more and it would vaporize, and any less and it wouldn't be boiling. When the pressure is low, it takes less energy for a molecule to jump into the empty space above the liquid, meaning that the temperature required to make the water boil is less. In a pressure cooker, the water molecule needs more energy to get into the crowded space above the liquid, so the temperature required to make it boil gets higher.

As for the reason of pressure being lower at higher altitude: imagine you are under a rug, and then imagine you are under ten rugs. the more layers of carpet on you, the higher the pressure. the atmosphere is no different. There is no air in space, and the only reason that earth has an atmosphere is because gravity pulls the air down to earth. The lower you altitude, the greater the thickness of the atmosphere above you, and the greater the pressure of the air weighing down on you. Conversely, the higher you go, the closer you are to space, and the less air there is above you weighing down on you.

Since the pressure at high altitude is low, water boils at a lower temperature. That means that the pot of boiling water at the top of Mt Whitney is significantly cooler than the pot of boiling water at the bottom of Death Valley, and cooler still than the boiling water in the pressure cooker. The pressure cooker makes it possible for the water to remain a liquid at higher temperatures, so you can make boiling water hotter than normally possible (the water can't vaporize because of the high pressure). Hence, due to the temperature differences, it would take longer to boil an egg at the top of a mountain, and less time to boil an egg in a pressure cooker.

Hi,

I am having a lot of trouble with calorimetry passages. The concept of bomb/coffee cup calorimeters doesn't seem so bad but I have a lot of trouble applying it to questions.

Hi, after going over the redox chapter, i basically understood everything, but I'm having problems with passages that deal with the application of redox reactions in real life. Say, if metal corrodes, how do you know if the metal is being reduced or oxidized? There was another passage where they did an experiment with metals in acids without any reduction potentials table, in this case, how do you know that they will react and that it's a redox reaction? and if so, that which one is reduced, and which one is oxidized (without any reduction potentials given)?

I am having a lot of trouble with calorimetry passages. The concept of bomb/coffee cup calorimeters doesn't seem so bad but I have a lot of trouble applying it to questions.

I will add calorimetry to the list of topics for the gen. chem. explanations thread. Basically, you need to remember that the heat lost by one substance is equal to the heat gained by another. Heat is a form of energy, so it should never be created nor destroyed.

Hi, after going over the redox chapter, i basically understood everything, but I'm having problems with passages that deal with the application of redox reactions in real life. Say, if metal corrodes, how do you know if the metal is being reduced or oxidized? There was another passage where they did an experiment with metals in acids without any reduction potentials table, in this case, how do you know that they will react and that it's a redox reaction? and if so, that which one is reduced, and which one is oxidized (without any reduction potentials given)?

During corrosion, the metals get oxidized and the oxygen gets reduced. The best place to start when considering any problem like this is to write down a reaction for what is occurring. This will help you reason out each partner's role in the redox reaction. Consider this generic reaction that is occurring for an oxidation of a metal:

M (s) + O2 (g) -> M^xO^y

where x and y are positive integers. Your metal starts out in its standard state (solid metal), which has an oxidation number = 0. The same is true for the oxygen gas. After reaction to form the metal oxide, your metal is now a cation and your oxygen now has a formal charge of -2. Thus, the metal has been oxidized, and the oxygen has been reduced.

In the case of metals being dissolved in acid, the acid ends up forming H2 gas. Try to write an equation like we did before, and you should find that again the metal gets oxidized while the H gets reduced:

M + H+ -> H2 + M^n+

where n is some positive integer.

im not sure if i can post this its examkrackers question of the day, just wondering why the answer isnt 2000 ml but 1900 ml

100 mL of 1 molar HCl is to be diluted to 0.05 molar HCl. How much water should be added?

im using the M1V1=M2V2 this damn formula never works it seems =p.

im not sure if i can post this its examkrackers question of the day, just wondering why the answer isnt 2000 ml but 1900 ml

100 mL of 1 molar HCl is to be diluted to 0.05 molar HCl. How much water should be added?

im using the M1V1=M2V2 this damn formula never works it seems =p.
V1 and V2 in that equation are the total volumes. You end up with V2 = 2000 ml, but you then must realize that they are asking for how much water to add to the 100 ml. Hence, you would add 2000 - 100 = 1900 ml.

Why does increasing the vapor pressure DECREASE the boiling point? I thought a liquid boiled when atmospheric pressure and the vapor pressure of the liquid were equal. So if the vapor pressure is somehow increased, how does that translate to a shift in the boiling point???

Why does increasing the vapor pressure DECREASE the boiling point? I thought a liquid boiled when atmospheric pressure and the vapor pressure of the liquid were equal. So if the vapor pressure is somehow increased, how does that translate to a shift in the boiling point???

I don't understand your question. If you raise the vapor pressure above the ambient or atmospheric pressure, then boiling will occur. But changing the vapor pressure doesn't change the boiling point, which is constant for a substance at a given temperature and pressure. Changing the atmospheric pressure does affect the boiling point, but not the way you described. Nutmeg's post above explains this relationship. You can also think of it this way: if you lower the atmospheric pressure, you have lowered the vapor pressure that you need to reach for boiling to occur. If you put liquid water into a vacuum, it will vaporize instantly. Why? Because there is no atmospheric pressure, so the vapor pressure of the water doesn't have to be very high for boiling to occur.

maybe phrased a bit differently,

if you lets say add NaCl to water, the boiling point increases to greater than 100 degrees celsius. Why does this then mean that the atmospheric pressure must be less than 1 atm?

maybe phrased a bit differently,

if you lets say add NaCl to water, the boiling point increases to greater than 100 degrees celsius. Why does this then mean that the atmospheric pressure must be less than 1 atm?

The atmospheric pressure has not changed! It is constant for a given set of conditions. What you've done by adding salt to the water is to lower the VAPOR PRESSURE of that water. Since you need to have your vapor pressure equal to the atmospheric pressure before boiling will occur, then you will need to boil your water at a higher temperature in order to raise the vapor pressure up to the level of the atmospheric pressure.

QofQuimica,

This probably is a stupid question but I was wondering if there are atoms that can have more than four bonds... greater than an octet??? Do we have to know these atoms that can have more than an octet??

I was just curious....

QofQuimica,

This probably is a stupid question but I was wondering if there are atoms that can have more than four bonds... greater than an octet??? Do we have to know these atoms that can have more than an octet??

I was just curious....

It's not a stupid question at all, and yes, there are. Elements in the third row of the periodic table and below can exceed their octets by hybridizing with their d-orbitals. For example, phosphorus can hybridize to form 5 sp3d orbitals, and have five bonds; this is called trigonal bipyramidal geometry. Sulfur can form 6 sp3d2 orbitals, have six bonds, and take on octahedral geometry. Note that second row elements can NEVER exceed their octets. So don't ever make pentavalent carbons, nitrogens, or oxygens!

It's not a stupid question at all, and yes, there are. Elements in the third row of the periodic table and below can exceed their octets by hybridizing with their d-orbitals. For example, phosphorus can hybridize to form 5 sp3d orbitals, and have five bonds; this is called trigonal bipyramidal geometry. Sulfur can form 6 sp3d2 orbitals, have six bonds, and take on octahedral geometry. Note that second row elements can NEVER exceed their octets. So don't ever make pentavalent carbons, nitrogens, or oxygens!


I just looked at the periodic table and I'm a little confused.. you say d orbitals for P and S but they are in their p orbital... I thought that they can fill up their p subshell by sharing electrons but how do they have a d subshell???

Thanks for clarifying this ... I got a question wrong do to this (it was a VSPER one)

I just looked at the periodic table and I'm a little confused.. you say d orbitals for P and S but they are in their p orbital... I thought that they can fill up their p subshell by sharing electrons but how do they have a d subshell???


They use their empty 3d orbitals. This is why the third row elements can do it, but the second row elements cannot (there are no 2d orbitals, right?)

For solubility guidelines (i.e. N03 soluble with NH4 and alkali metals). Did you suck it up and memorize them or only a few or have a good mnemonic?

For solubility guidelines (i.e. N03 soluble with NH4 and alkali metals). Did you suck it up and memorize them or only a few or have a good mnemonic?

Solubility mnemonic (http://forums.studentdoctor.net/showpost.php?p=2780129&postcount=6)

Another question about delta H values from Kap's FL 3

Calculate the delta H of formation of Fe2O3

6Fe2O3 --> 4 Fe3O4 + O2 +472 kJ/mol
3Fe + 2O2 ---> Fe3O4 -1118.4 kj/mol

Choices are in kj/mol

667
-1559
-824
-667

The answer is C, but I had no clue how they got it.

Another question about delta H values from Kap's FL 3

Calculate the delta H of formation of Fe2O3

6Fe2O3 --> 4 Fe3O4 + O2 +472 kJ/mol
3Fe + 2O2 ---> Fe3O4 -1118.4 kj/mol

Choices are in kj/mol

667
-1559
-824
-667

The answer is C, but I had no clue how they got it.

This is a Hess's Law problem. Hess's Law tells you that since enthalpy is a state function, we don't care about the path taken to go from starting materials (Fe metal and O2 gas) to product (Fe2O3). Start by writing out the net equation of the reaction for formation of Fe2O3 from its constituent elements. You then need to manipulate the step equations (given in the problem) so that the intermediate (Fe3O4) will cancel out and give you the net equation. Don't forget to multiply the deltaH value by the same amount as its equation. Also, you will have to turn one equation backwards (multiply by negative one). Give it a shot yourself, and let me know if you need more help.

This is a Hess's Law problem. Hess's Law tells you that since enthalpy is a state function, we don't care about the path taken to go from starting materials (Fe metal and O2 gas) to product (Fe2O3). Start by writing out the net equation of the reaction for formation of Fe2O3 from its constituent elements. You then need to manipulate the step equations (given in the problem) so that the intermediate (Fe3O4) will cancel out and give you the net equation. Don't forget to multiply the deltaH value by the same amount as its equation. Also, you will have to turn one equation backwards (multiply by negative one). Give it a shot yourself, and let me know if you need more help.

Q, your knowledge of the sciences is amazing. You knock down every question. Keep up the good work. :thumbup:

for anode and cathode, why is there a diff between the physics anode and cathode, and the chemistry anode and cathode? they always change the signs .. why?

for anode and cathode, why is there a diff between the physics anode and cathode, and the chemistry anode and cathode? they always change the signs .. why?

Because physicists solve these problems from the perspective of current flow, which is positive, and chemists solve them from the perspective of electron flow, which is negative. I don't know why the conventions are different, but I've always thought that the chemistry one makes more sense, since current flows in the direction of imaginary positive charges, as opposed to ostensibly real electrons. But I may be somewhat biased. ;)

I finished studying all of EK and I have the basics down for the most part but there are a number of things mentioned in the AAMC MCAT Topic List that were not covered in EK materials. Could you comment on what we need to know about some of these. Thanks.

bonding-the ionic bond(electrostatic forces between ions)
E=lattice energy
and
Force attraction=R(n+e)(n-e)/(dxd)

How many moles of NaOH must be added to 1.0L of 2.0M acetic acid to produce a buffered solution at pH=pKa?

The answer is 1, but I'm having a hard time buying the stmt:
[C2H3O2-] + [HC2H3O2] = 2.0M

:confused:

I finished studying all of EK and I have the basics down for the most part but there are a number of things mentioned in the AAMC MCAT Topic List that were not covered in EK materials. Could you comment on what we need to know about some of these. Thanks.

bonding-the ionic bond(electrostatic forces between ions)
E=lattice energy
and
Force attraction=R(n+e)(n-e)/(dxd)

I would say you don't need to know about any of these things in great detail, and you probably already know most of this anyway.

Lattice energy refers to the energy of attraction between the cations and anions of a salt; the greater this attaction is, the higher the lattice energy will be. You can also think of it as the energy required to pull the ions of a salt apart from one another. Smaller, highly charged ions tend to form salts with greater lattice energies. Salts with higher lattice energies tend to be less soluble because the ions don't want to separate. You can use Coulomb's Law (same as from physics) to calculate the ionic force between cations and anions if you know the van der Waals radii of the ions and the charges on the ions.

How many moles of NaOH must be added to 1.0L of 2.0M acetic acid to produce a buffered solution at pH=pKa?

The answer is 1, but I'm having a hard time buying the stmt:
[C2H3O2-] + [HC2H3O2] = 2.0M


You start out with 1 L x 2 mol/L = 2 moles of acid (HA). So to reach the pKa, you have to add one mole of NaOH, leaving you with one mole of conjugate acid (HA) and one mole of conjugate base (A-). Since you have one liter of solution, then you end up with 1 mol/L of HA and 1 mol/L of A-. Add the two individual molarities to get the total molarity of particles....though I'm a little confused myself about what the point was in doing that. Were they trying to prove to you that the total molarity stays constant?

Because physicists solve these problems from the perspective of current flow, which is positive, and chemists solve them from the perspective of electron flow, which is negative. I don't know why the conventions are different, but I've always thought that the chemistry one makes more sense, since current flows in the direction of imaginary positive charges, as opposed to ostensibly real electrons. But I may be somewhat biased. ;)


thanks for the response.

so does that mean chemist make the anode plus and the cathode minus, while physicist make it the opposite? so does that mean chemist make it more like an electrolytic cell? im confused... can you give me some examples?

Hi Q - you're the best! Thanks for taking for everything you do for us. I guess the part I was having difficulty was with the statement that the sum of the concentration of acetic acid + it's conjugate = 2.0 M which is the stated conc. of acetic acid (just by itself)

I guess one way to look at it is that acetic acid could exist in an aq soln as it's conjugate [A-] & [HA] so total conc is the sum of the two? The other reason I was having a hard time with this is because when I use the H/H eqn:
pH = pKa + log(base/acid)
since pKa = pH, thus
base/acid = 1

Now my point is this can mean that moles of base = moles of acid (apparently a true statement). But it could also mean [base]/[acid] = 1 in which case [A-] = [HA] and so how can [HA] + [A-] = 2??

Help! :scared:


You start out with 1 L x 2 mol/L = 2 moles of acid (HA). So to reach the pKa, you have to add one mole of NaOH, leaving you with one mole of conjugate acid (HA) and one mole of conjugate base (A-). Since you have one liter of solution, then you end up with 1 mol/L of HA and 1 mol/L of A-. Add the two individual molarities to get the total molarity of particles....though I'm a little confused myself about what the point was in doing that. Were they trying to prove to you that the total molarity stays constant?

thanks for the response.

so does that mean chemist make the anode plus and the cathode minus, while physicist make it the opposite? so does that mean chemist make it more like an electrolytic cell? im confused... can you give me some examples?

I meant that the sign of the flow of charge, and therefore its direction, is reversed. For a galvanic (spontaneous) cell, a chemist would say that the electrons move from the negative anode to the positive cathode (reduction always occurs at the cathode, so the electrons always go there). A physicist would say that the current flows from the positive cathode to the negative anode. So the electrode signs should be the same, but the flow of charges is reversed, since the signs of the charges of the electrons and the current are opposite. If the electrode signs change, that means you have an electrolytic (nonspontaneous) cell; those cells have a positive anode and a negative cathode.

The other reason I was having a hard time with this is because when