General Chemistry Question Thread

[QofQuimica]

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.

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[QofQuimica]

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.)

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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

 

[QofQuimica]

(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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[freesolo]

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

 

[jmnykrkts]

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

 

[QofQuimica]

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)

 

[QofQuimica]

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.

 

[freesolo]

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?

 

[QofQuimica]

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.

 

[Shrike]

Yes, that is correct.

By the way: why?

(serious question, not rhetorical)

 

[Nutmeg]

By the way: why?

(serious question, not rhetorical)

*

 

[freesolo]

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?

 

[Learfan]

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.

 

[Sparky Man]

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

 

[Shrike]

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

 

[QofQuimica]

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

lol, this will not be on the MCAT.

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.

 

[Shrike]

lol, this will not be on the MCAT.

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?

 

[Nutmeg]

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.

 

[Shrike]

The wavelength of light... to be bleached.

Bravo, and bless the new forum. Thank you.

 

[Sparky Man]

...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.

 

[Kussemek]

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....

 

[LT2]

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!

 

[Blackstars]

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

 

[psiyung]

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

 

[MDhopeful023]

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!

Hopeful023

 

[QofQuimica]

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.

 

[MDhopeful023]

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.

 

[perfectmoment]

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

 

[Belfagor]

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

 

[Belfagor]

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?

 

[frany584]

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?

 

[Sparky Man]

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

 

[Nitya2284]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[freesolo]

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

 

[freesolo]

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

 

[frany584]

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

 

[Nutmeg]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[Futuredoctr]

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.