bme post

[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

*********

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.

---

Members don't see this ad.

 

[QofQuimica]

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.

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.

 

[gujuDoc]

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!

 

[QofQuimica]

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.

 

[docmd2010]

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.

 

[Emann8]

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

 

[AggieJohn]

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

 

[el.harpo]

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?

 

[Shrike]

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).

 

[stoleyerscrubz]

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!

 

[QofQuimica]

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.)

 

[hippocampus]

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.

 

[QofQuimica]

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.

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.

 

[QofQuimica]

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

 

[Mutt]

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....?

 

[hippocampus]

wuts the diff bt extensive and intensive property?

 

[Nutmeg]

wuts the diff bt extensive and intensive property?

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.

 

[frankrizzo18]

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]

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.

 

[QofQuimica]

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.

 

[hippocampus]

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?

 

[Mutt]

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....??

 

[stoleyerscrubz]

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

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[Andrew99]

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?

 

[stoleyerscrubz]

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,

 

[QofQuimica]

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

 

[QofQuimica]

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

 

[bnoosha]

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.

 

[QofQuimica]

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:

Ksp post 1

Ksp post 2

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

 

[hippocampus]

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!

 

[Twitch]

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

 

[MarzMD]

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?

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[QofQuimica]

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.

 

[Twitch]

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.

 

[QofQuimica]

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.

 

[MarzMD]

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.

 

[Nitya2284]

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..

 

[QofQuimica]

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.

 

[QofQuimica]

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).

 

[Nitya2284]

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?

 

[QofQuimica]

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.

 

[Tracy47]

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!

 

[Nutmeg]

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

 

[Nutmeg]

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.

 

[faluri]

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.

 

[Tracy47]

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)?