AAMC CBT7 and 7R OFFICIAL Q&A

[Vihsadas]

This is the official Q&A thread for AAMC CBT7 and 7R.

Please post ONLY questions pertaining to AAMC CBT7 and 7R.
Out of respect for people who may not have completed the other exams, do not post questions or material from any other AAMC exam.

Please see this thread for the rules of order before you post.

Good luck on your MCAT!

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[dmplz707]

Hey guys for this question I had it down to A & B but why wouldn't it be A? Thanks!

H. pylori infection may cause increased proliferation of mucosal cells in the stomach. This may lead to gastric cancer if:

A genetic mutations occur in proliferating germ cells.

B genetic mutations occur in proliferating somatic cells.

C the immune system fails to recognize bacterial antigens.

D crowded mucosal cells are likely to remain in interphase.

 

[Forester]

Hey guys for this question I had it down to A & B but why wouldn't it be A? Thanks!

H. pylori infection may cause increased proliferation of mucosal cells in the stomach. This may lead to gastric cancer if:

A genetic mutations occur in proliferating germ cells.

B genetic mutations occur in proliferating somatic cells.

C the immune system fails to recognize bacterial antigens.

D crowded mucosal cells are likely to remain in interphase.

It wouldn't be A because it says it infects the mucosal cells of the stomach. Germ cells give rise to the gametes of the organism. Somatic cells give rise to all other cells. This question comes up a lot in practice tests. Make sure you know the distinction.

 

[dmplz707]

Hey guys thanks for all the help so far. I was wondering if someone can provide a better explanation to question 42 as I still cannot seem to understand the answer explanation.

Item 42

Before the Landers quake, coincidence was argued as a sufficient explanation for what is now believed to be triggered-quake events. For these earlier events, which of the following does NOT support the coincidence hypothesis?

A The timing of subsequent quakes provided ambiguous evidence.

B Generally, the distances to the subsequent quakes was excessive.

C Too few of the subsequent quakes were recorded to establish a clear connection.

D Generally, the subsequent quakes were scattered in all directions.
Since we would expect the quake information to propagate in all directions, it does not support the coincidence hypothesis to learn that subsequent quakes occurred in all directions. The other three answers all stress the possibility of coincidence.

For this question could someone please show me how they derived (d2/d1)^3? Thanks.

Item 43

Solution
What is the ratio of lasting deformations produced by a quake at 16 L from its center to those produced at 4 L from its center?

A 1/16

B 1/32

C 1/64
Lasting deformations go as (L/d)3 according to the passage. Thus for two different values of d, d1 and d2, the ratio of lasting deformations will go as (d2/d1)3. This value will be (4/16)3 = (1/4)3 = 1/64. This is answer C.

D 1/128

138

If the signaling interaction at the two-cell stage involves protein, then how could the actinomycin-treated embryo develop normally? If mRNA synthesis is inhibited then where exactly will the required protein come from? The oleosome ?

Thank you so much guys!

 

[tak08810]

Could someone please explain the reasoning for question #138? I believe no one has addressed this question even though it is old. Thanks!

I'm utterly baffled by this question as well. If there's no transcription, meaning there's no rRNA and mRNA, how on earth are there proteins? LMFAO

And yeah I don't really get

"Hey guys thanks for all the help so far. I was wondering if someone can provide a better explanation to question 42 as I still cannot seem to understand the answer explanation.

Item 42

Before the Landers quake, coincidence was argued as a sufficient explanation for what is now believed to be triggered-quake events. For these earlier events, which of the following does NOT support the coincidence hypothesis?

A The timing of subsequent quakes provided ambiguous evidence.

B Generally, the distances to the subsequent quakes was excessive.

C Too few of the subsequent quakes were recorded to establish a clear connection.

D Generally, the subsequent quakes were scattered in all directions."

Either. If the subsequent quakes were scattered in all directions wouldn't that be evidence of their randomness?

 

[muhali3]

Why does C8 have higher priority than C6 when designating chirality for C7?

An oxygen molecule is 2 carbons away on both sides. Do ketones have higher priority than epoxides in Cahn-Ingold-Prelog? I thought only the MW of the atom counted?

 

[notbobtrustme]

Between the 'x' and the circled Carbon with the H going out of page? That's a stereocenter? Does it have four different substituents? I see:
1. CH
2. CH
3. CH2
4. H

I've always been sliding by when it comes to ID'ing stereocenters and such, how would I go about this one?

That's not a good way to determine if a carbon is a stereocenter. You take each bond and trace a path out. If you have a different path, then the two substituents are not identical, therefore, that carbon is a stereocenter.

 

[notbobtrustme]

Why does C8 have higher priority than C6 when designating chirality for C7?

An oxygen molecule is 2 carbons away on both sides. Do ketones have higher priority than epoxides in Cahn-Ingold-Prelog? I thought only the MW of the atom counted?

You count the carbonyl twice, whereas you only count the hydroxyl once.

 

[AugustMCAT]

Which of the following alkyl halides is most readily prepared by a reaction between the corresponding alcohol and concentrated HCl?

A. Isopropyl chloride
B. Methyl Chloride
C. Sec-butyl Chloride
D. Tert-butyl chloride

Now the rule with amines is that the more donating groups, the more basic the amine. This obviously applies here too, so I marked choice D, which is the correct answer. However, when reviewing my answers with m extra time, I changed it to choice C. Why? A tertiary alcohol is sterically hindered, so it seems like its basicity would actually DECLINE, much like amines. So why is D correct over C?

 

[AugustMCAT]

I'm utterly baffled by this question as well. If there's no transcription, meaning there's no rRNA and mRNA, how on earth are there proteins? LMFAO

And yeah I don't really get

"Hey guys thanks for all the help so far. I was wondering if someone can provide a better explanation to question 42 as I still cannot seem to understand the answer explanation.

Item 42

Before the Landers quake, coincidence was argued as a sufficient explanation for what is now believed to be triggered-quake events. For these earlier events, which of the following does NOT support the coincidence hypothesis?

A The timing of subsequent quakes provided ambiguous evidence.

B Generally, the distances to the subsequent quakes was excessive.

C Too few of the subsequent quakes were recorded to establish a clear connection.

D Generally, the subsequent quakes were scattered in all directions."

Either. If the subsequent quakes were scattered in all directions wouldn't that be evidence of their randomness?

No, because remember, when the impulses propagate outwards, they are going out in a circular fashion. So the fact that the quakes were scattered supports both hypotheses equally.

 

[AugustMCAT]

Could someone please explain the reasoning for question #138? I believe no one has addressed this question even though it is old. Thanks!

The signaling MOST involves which classes of molecules. Could rRNA be involved? Sure. But use your intuition here. Signaling will be affected by the far larger factor of untranslated mRNA (aka proteins).

 

[destroyMCAT]

PS #11: Why is the answer not coulomb forces pulling on the electron sea?

 

[Bumbl3b33]

PS #11: Why is the answer not coulomb forces pulling on the electron sea?

This one almost got me. If you read the question carefully, it asks about going from B to C, not C to B. It's able to go past B w/o being stuck b/c of f=kqq/r^2 because it has gained so much velocity (and thus momentum) as it 'fell' from A to B, that it just zooms past B and to C. Does that make sense? I can explain it another way, if need be.

 

[destroyMCAT]

This one almost got me. If you read the question carefully, it asks about going from B to C, not C to B. It's able to go past B w/o being stuck b/c of f=kqq/r^2 because it has gained so much velocity (and thus momentum) as it 'fell' from A to B, that it just zooms past B and to C. Does that make sense? I can explain it another way, if need be.

Yes, I see it now. Thanks!

 

[destroyMCAT]

Hm. What about PS #18. How can you do fractional distillation on a gas? I know that the components have to have different boiling points, but usually aren't fractional distillation applied to liquids? And since it's a gas, it has already reached it's boiling point..

 

[AugustMCAT]

Which of the following alkyl halides is most readily prepared by a reaction between the corresponding alcohol and concentrated HCl?

A. Isopropyl chloride
B. Methyl Chloride
C. Sec-butyl Chloride
D. Tert-butyl chloride

Now the rule with amines is that the more donating groups, the more basic the amine. This obviously applies here too, so I marked choice D, which is the correct answer. However, when reviewing my answers with m extra time, I changed it to choice C. Why? A tertiary alcohol is sterically hindered, so it seems like its basicity would actually DECLINE, much like amines. So why is D correct over C?

Bump, MCAT in 3 days!

 

[Bumbl3b33]

Bump, MCAT in 3 days!

Sterically hindered? I think you're thinking this is going to be an E1 or SN2 reaction, but it's not, it's a nucleophilic attack (I think...I don't know, I don't really look at names and stuff, just think about how the reaction would happen). First, "readily prepared" = fastest reaction. Let's look at the mechanism. So basically what happens is that the HCl is a strong acid, and because it is a strong acid it wants to get rid of it's proton, and when it does, the only thing that will accept it is the oxygen on the alcohol. So the oxygen gets protonated and becomes a hydronium ion; once it becomes a hydronium ion, it breaks off because it's a good/better leaving group. once it leaves, a carbocation is made on the carbon that was connected to the water that left. So now it's a matter of "which compound makes the best carbocation" because the more stable the carbocation, the faster the reaction occurs and when you look at your answer choices in their carbocation form, meaning sans chloride, you see that the tertbutyl group is the only tertiary structure, thereby making it the most stable carbocation. Did that make sense? I can be less verbose, I swear! lol

 

[AugustMCAT]

Sterically hindered? I think you're thinking this is going to be an E1 or SN2 reaction, but it's not, it's a nucleophilic attack (I think...I don't know, I don't really look at names and stuff, just think about how the reaction would happen). First, "readily prepared" = fastest reaction. Let's look at the mechanism. So basically what happens is that the HCl is a strong acid, and because it is a strong acid it wants to get rid of it's proton, and when it does, the only thing that will accept it is the oxygen on the alcohol. So the oxygen gets protonated and becomes a hydronium ion; once it becomes a hydronium ion, it breaks off because it's a good/better leaving group. once it leaves, a carbocation is made on the carbon that was connected to the water that left. So now it's a matter of "which compound makes the best carbocation" because the more stable the carbocation, the faster the reaction occurs and when you look at your answer choices in their carbocation form, meaning sans chloride, you see that the tertbutyl group is the only tertiary structure, thereby making it the most stable carbocation. Did that make sense? I can be less verbose, I swear! lol

That made total sense! This takes me back to an EK passage.

So readily prepared = fastest...that is the key. The reaction is SN1 for the tertiary carbocation, making the rate-limiting step the protonation of the alcohol and water leaving. This occurs relatively fast compared to an SN2 mechanism, which is what takes place for everything except the tertiary and MAYBE the secondary. This is because the Cl- atoms will collide and sometimes form the product, and sometimes form nothing. So in comparing the protonation step to SN2, protonation actually occurs faster, so the SN1/tertiary alcohol is best.

1. How did you know "readily prepared" was kinetics?

2. Am I correct in assuming that some reactions are SN1 (tertiary...MAYBE secondary, but still slower than tert) vs SN2 (3 other answer choices)?

 

[Bumbl3b33]

That made total sense! This takes me back to an EK passage.

So readily prepared = fastest...that is the key. The reaction is SN1 for the tertiary carbocation, making the rate-limiting step the protonation of the alcohol and water leaving. This occurs relatively fast compared to an SN2 mechanism, which is what takes place for everything except the tertiary and MAYBE the secondary. This is because the Cl- atoms will collide and sometimes form the product, and sometimes form nothing. So in comparing the protonation step to SN2, protonation actually occurs faster, so the SN1/tertiary alcohol is best.

1. How did you know "readily prepared" was kinetics?

2. Am I correct in assuming that some reactions are SN1 (tertiary...MAYBE secondary, but still slower than tert) vs SN2 (3 other answer choices)?

1. Well, how fast something is produced is dependent on how quickly it will go through the reaction right? Well think of it this way, if you have your positive charge 'spilled' all over the place, it's going to be harder for the negative charge to come in and attack it, right? So now you can go back to your initial thinking about the inductive effect and say, well the more substituted my carbocation is with alkyl groups, the more they're going pushing electrons towards themselves, thereby repelling the positive charge to the center--this is what people mean when they say 'more substituted means better carbocation stability'. This concentration/stabilization of positive charge is more effective with more akyl groups so, more substituted = higher chance/better attack by nucleophile (the Cl-), and so the faster the product forms!

2. I'm not sure if I completely understand this question but--- SN1 is impossible in methyl and SN2 is impossible with tertiary. No arguments, not stable enough of a carbocation, and too much steric hindrance, respectively. If you clarify your question, I'd be more than happy to shed more light if I can!

 

[AugustMCAT]

1. Well, how fast something is produced is dependent on how quickly it will go through the reaction right? Well think of it this way, if you have your positive charge 'spilled' all over the place, it's going to be harder for the negative charge to come in and attack it, right? So now you can go back to your initial thinking about the inductive effect and say, well the more substituted my carbocation is with alkyl groups, the more they're going pushing electrons towards themselves, thereby repelling the positive charge to the center--this is what people mean when they say 'more substituted means better carbocation stability'. This concentration/stabilization of positive charge is more effective with more akyl groups so, more substituted = higher chance/better attack by nucleophile (the Cl-), and so the faster the product forms!

2. I'm not sure if I completely understand this question but--- SN1 is impossible in methyl and SN2 is impossible with tertiary. No arguments, not stable enough of a carbocation, and too much steric hindrance, respectively. If you clarify your question, I'd be more than happy to shed more light if I can!

1. Gotcha!

2. That is exactly what I'm saying . I am assuming the tertiary undergoes SN1, while all others undergo SN2. Since the rate-limiting step (protonation) of the SN1 reaction is faster than the SN2 reaction (which has many collisions that don't produce new molecules), the tertiary acts the fastest. If I were to take this a step further...

Tertiary(SN1)>Secondary(can do SN1)>Methyl(SN2)>Primary(SN2)

Speed-wise. Am I correct in making these assumptions? Am I overthinking things? If so, is there a general rule you suggest for balancing out the effects of steric hindrance vs donating group for acidity/nucleophilicity?

 

[Bumbl3b33]

1. Gotcha!

2. That is exactly what I'm saying . I am assuming the tertiary undergoes SN1, while all others undergo SN2. Since the rate-limiting step (protonation) of the SN1 reaction is faster than the SN2 reaction (which has many collisions that don't produce new molecules), the tertiary acts the fastest. If I were to take this a step further...

Tertiary(SN1)>Secondary(can do SN1)>Methyl(SN2)>Primary(SN2)

Speed-wise. Am I correct in making these assumptions? Am I overthinking things? If so, is there a general rule you suggest for balancing out the effects of steric hindrance vs donating group for acidity/nucleophilicity?

Secondary can do Sn2 or Sn1, depending on your solvent, and the leaving group. I wouldn't worry too much about this, I've never seen an OChem question that really harps on this line, unless there are data in the passage. As for speed, just remember that the more stable intermediate leads to a faster reaction.

 

[destroyMCAT]

Can someone explain the application of the physics concept on passage 7 of the PS? Particularly #43,44, and 46. It feels like this passage is more reading comprehension than the other passages, but then again, I could be wrong..

 

[Bumbl3b33]

Can someone explain the application of the physics concept on passage 7 of the PS? Particularly #43,44, and 46. It feels like this passage is more reading comprehension than the other passages, but then again, I could be wrong..

This passage actually required very little 'physics' in the standard way.

#43 = set up a ratio, the passage says that it's L/d^3. and question gives you 2 different d's

#44-- where would you expect an earthquake to happen if only a little jolt in the plates caused a big movement on the plates? The place where there's already a lot of pressure and all tht was needed was a little energy to push it over to a full blown earthquake, right?

#46-- this is just asking what will be the doppler effect of a moving object north. well, if it's moving north, you know the waves infront of it (facing north) will be shorter than those behind it (south)

here's a pictorial representation of the doppler effect
http://www.google.com/imgres?q=dopp...9&tbnw=212&start=0&ndsp=15&ved=1t:429,r:9,s:0

 

[Reine]

Hey all,

I'm having a bit of trouble understanding the concept behind PS #35.

I see that yeah, there's less repulsion between them... but the Keq is also lower for the second reaction. If there's less repulsion between the two reactants, shouldn't it have gone up?

Thanks! And hope everyone else is doing well

 

[BilalL]

Hey all,

I'm having a bit of trouble understanding the concept behind PS #35.

I see that yeah, there's less repulsion between them... but the Keq is also lower for the second reaction. If there's less repulsion between the two reactants, shouldn't it have gone up?

Thanks! And hope everyone else is doing well

Keq doesn't necessarily give you the reaction rate. It just tells you generally how many products there are versus reactants at equilibrium. So for rxn 1, the keq is much higher but it takes much longer to reach versus getting to the keq in rxn 2.

The passage gives you 1 reason rxn 2 is faster (the proton withdraws electron density from oxygen making the O-Cl bond slightly weaker), but the other reason must be the ionic repulsion of the like negative charges on the reactants which would disfavor a reaction.

So there are two things to consider.
One is how much product vs reactant there is at equilibrium and the other is how fast equilibrium was reached.

 

[ljc]

Can anyone explain #208?
In the key, it says "A negative entropy of activation indicates that the order of the transition state is INCREASING" (emphasis mine)
This seems backwards to me? Why would a negative entropy value correspond to increasing entropy?
Wikipedia seems to agree:
Positive values suggest that entropy increases upon achieving the transition state, which often indicates a dissociative mechanism.

Am I missing something?

 

[Ssina]

Just take the word "order" in there.... Whenever order increases, basically disorder decreases... Thats a negative change in entropy. (basically it's saying order increases ; entropy= DISorder)

In wiki, it's saying the opposite that positive is increase in entropy (or disorder)

 

[ljc]

Well that was dumb of me. Thanks!

 

[Erythropoietin]

Yea that's confusing for most people.
Increase in disorder = positive S
Increase in order = negative S

 

[princesslinda]

This a question in regards to #35 on cbt

In addition to the explanation in the passage, the rate of reaction 2 is different from the rate of reaction 1 because the formation of HOCl by the protonation of the oxygen:

Answer: reduces the electronic repulsion forces between the reactants.

I put "reduces the electronic attraction forces between the chlorine atom and the oxygen atom that is being transferred" because in the passage it states that the hydrogen atom reduces the charge on the oxygen aton, facilitating the breaking of the O-Cl bond. Are these two statements not the same?

 

[wanderedtoolong]

I chose the same answer on #35- can anyone explain why it's not correct?

 

[MedPR]

This a question in regards to #35 on cbt

In addition to the explanation in the passage, the rate of reaction 2 is different from the rate of reaction 1 because the formation of HOCl by the protonation of the oxygen:

Answer: reduces the electronic repulsion forces between the reactants.

I put "reduces the electronic attraction forces between the chlorine atom and the oxygen atom that is being transferred" because in the passage it states that the hydrogen atom reduces the charge on the oxygen aton, facilitating the breaking of the O-Cl bond. Are these two statements not the same?

I chose the same answer on #35- can anyone explain why it's not correct?

The passage straight out tells you that reaction 2 is faster than reaction 1. Increasing repulsion and/or reducing attraction (answers B, C, and D) are not consistent with the passage information that reaction 2 is faster. Answer A is the only valid option.

 

[MrNeuro]

The passage straight out tells you that reaction 2 is faster than reaction 1. Increasing repulsion and/or reducing attraction (answers B, C, and D) are not consistent with the passage information that reaction 2 is faster. Answer A is the only valid option.

i still don't buy this answer
D is consistent with the passage as it states

"HOCl allows the oxygen transfer to take place more quickly because the hydrogen atom reduces the carge on the oxygen atom, facilitating the breaking of the O-Cl blond."

aaaaaahhhhhhh but i think i have the correct reasoning now

the reason A is a better choice is that the next sentence states

"Reaction 2 is first order in both NO2 - and HOCl" meaning it has 2nd order kinetics and hence is associative hence it has to do w/ the reactants associating together

 

[dennis-brodmann]

This question pertains to Passage III about the purification of silicon.

Which of the following elements could best substitute for potassium in the purification of silicon?

A) H2
B) Na
C) Mg
D) Ca

In the passage, they mentioned that potassium was used in purification, but in the given reactions, we see that H2 is involved.

I chose A, but the answer is B.

I understand the concept they were trying to test was that chemicals in the same group have the same chemical properties, but I don't understand what makes A incorrect if hydrogen gas is involved in the reactions.

 

[dennis-brodmann]

Bump

 

[The Heisenberg]

This question pertains to Passage III about the purification of silicon.

Which of the following elements could best substitute for potassium in the purification of silicon?

A) H2
B) Na
C) Mg
D) Ca

In the passage, they mentioned that potassium was used in purification, but in the given reactions, we see that H2 is involved.

I chose A, but the answer is B.

I understand the concept they were trying to test was that chemicals in the same group have the same chemical properties, but I don't understand what makes A incorrect if hydrogen gas is involved in the reactions.

If you look at the last sentence, it says " Further purification is achieved by halogenating the silicon, purifying the resulting gas by fractional distillation, and then REDUCING the halogenated silicon compound (Equations 2-3)."

Basically, equation 3 has nothing to do with the preliminary purifying step. I'm thinking it is part of the fractional distillation, and reducing part of the experiment.

 

[dennis-brodmann]

If you look at the last sentence, it says " Further purification is achieved by halogenating the silicon, purifying the resulting gas by fractional distillation, and then REDUCING the halogenated silicon compound (Equations 2-3)."

Basically, equation 3 has nothing to do with the preliminary purifying step. I'm thinking it is part of the fractional distillation, and reducing part of the experiment.

Thanks Heisenberg!

 

[dennis-brodmann]

I really do not understand how one can say that if "some people are psychologically incapable of breaking their promises" that would challenge the author's assertion that it is impossible to surrender responsibility. I feel like this a huge leap in reasoning.

Can anyone please explain?

 

[Hirwipi]

For PS #33:

Can someone please explain this answer? I don't see anything in my prep books about pH and rate laws, and the AAMC explanation isn't helping... Thank you!

Reaction:
(CLO3-) + (6Br-) + (6H+) --> (Cl-) + (3Br2) + 3H20

Question:
If the rate of formation of Cl- in Reaction were 1.0x10^-2 M/sec at a pH of 1, what would it be at a pH of 2? (Note: assume that other conditions are identical).

a) 1 x 10-1 M/s
b) 1 x 10^-2 M/s
c) 2 x 10^-2 M/s
d) 1 x 10^-4 M/s

The answer was D.

Explanation from AAMC:
The rate law for Reaction is second order with respect to H ions. At a pH of 1 the H ion concentration is 1 x 10^-1. When the pH is increased to 2, the H ion concentrations is 1 x 10^-2. Because all other conditions remain the same, the rate is decreased: (1 x 10^-2)^2/(1 x 10^1)^2, or 1 x 10^-2 times. The rate would therefore be 1 x 10^-4 M/s.

 

[Patassa]

For PS #33:

Can someone please explain this answer? I don't see anything in my prep books about pH and rate laws, and the AAMC explanation isn't helping... Thank you!

Reaction:
(CLO3-) + (6Br-) + (6H+) --> (Cl-) + (3Br2) + 3H20

Question:
If the rate of formation of Cl- in Reaction were 1.0x10^-2 M/sec at a pH of 1, what would it be at a pH of 2? (Note: assume that other conditions are identical).

a) 1 x 10-1 M/s
b) 1 x 10^-2 M/s
c) 2 x 10^-2 M/s
d) 1 x 10^-4 M/s

The answer was D.

Explanation from AAMC:
The rate law for Reaction is second order with respect to H ions. At a pH of 1 the H ion concentration is 1 x 10^-1. When the pH is increased to 2, the H ion concentrations is 1 x 10^-2. Because all other conditions remain the same, the rate is decreased: (1 x 10^-2)^2/(1 x 10^1)^2, or 1 x 10^-2 times. The rate would therefore be 1 x 10^-4 M/s.

Sorry I was looking at the wrong problem

In this one, they tell you that the reaction is 2nd order with respect the concentration of H+, that means if your H+ concentration doubles, you'd see a 2^2= 4 times increase in product formation rate. Or in this case, your H+ concentration dropped by 10 (1 ph change = 10 units), therefore a 10^2=100 decrease in formation rate.

 

[Saharon]

Hi! I had a question on #147 (During the initial skin diving session, when her heart and breathing rates were increased, Sarah noticed that she produced more urine than usual. This was most probably a result of...). If Sarah's HR, Breathing rate, and BP were increased, why would her urine output be greater than normal? Isn't increased BP due to increased water retention and thus decreased urine output? Also, wouldn't the stimulation of the sympathetic nervous system also lead to decreased urine output?

 

[VitaminVater]

Aamc #7 bs #95

So where it asks how many possible stereoisomers for compound 1 I put 4 because the only 2 chiral centers which could each have 2 different orientations were the carbon pointed at by x and the one right below it.

Now the answer is 16 because apparently it also counts the 2 carbons on the ring bridge(with the H or CH3 group sticking out), and I did not count those because they already had their orientation assigned.
So I'm wondering, if this is ever asked again, do I always ignore if a certain chiral center already has an orientation assigned(wedge/dash) and count it as a chiral center(add 1 more to n)?

 

[gettheleadout]

Aamc #7 bs #95

So where it asks how many possible stereoisomers for compound 1 I put 4 because the only 2 chiral centers which could each have 2 different orientations were the carbon pointed at by x and the one right below it.

Now the answer is 16 because apparently it also counts the 2 carbons on the ring bridge(with the H or CH3 group sticking out), and I did not count those because they already had their orientation assigned.
So I'm wondering, if this is ever asked again, do I always ignore if a certain chiral center already has an orientation assigned(wedge/dash) and count it as a chiral center(add 1 more to n)?

Never ignore stereocenters that are depicted with a particular configuration already given. Those individuals configurations could still be different in a given stereoisomer and so should still be counted.

 

[Synapsis]

Aamc #7 bs #95

So where it asks how many possible stereoisomers for compound 1 I put 4 because the only 2 chiral centers which could each have 2 different orientations were the carbon pointed at by x and the one right below it.

Now the answer is 16 because apparently it also counts the 2 carbons on the ring bridge(with the H or CH3 group sticking out), and I did not count those because they already had their orientation assigned.
So I'm wondering, if this is ever asked again, do I always ignore if a certain chiral center already has an orientation assigned(wedge/dash) and count it as a chiral center(add 1 more to n)?

Yea I say you're just overthinking this a bit. Literally just do an unbias count of chiral centers , and do 2^n for max number of stereoisomers. You don't have to add or subtract things from the one already there.

It's like if someone asks how many people on my basketball team are on the court at any given time. I don't say "4, discounting me". I just say 5, because I'm naturally included. Similarly, if I have A and B, and B is a stereoisomer of A, I say I have two stereoisomers, not one.

?

 

[VitaminVater]

Yea I say you're just overthinking this a bit. Literally just do an unbias count of chiral centers , and do 2^n for max number of stereoisomers. You don't have to add or subtract things from the one already there.

It's like if someone asks how many people on my basketball team are on the court at any given time. I don't say "4, discounting me". I just say 5, because I'm naturally included. Similarly, if I have A and B, and B is a stereoisomer of A, I say I have two stereoisomers, not one.

?

Synapsis: teaches troubled,high-risk urban youth how to master the MCAT through basketball metaphors.

No but seriously thanks lol, it just pissed me off that I got such an easy question wrong, especially an ORGANIC chem question ughh.

 

[sakabato93]

Synapsis: teaches troubled,high-risk urban youth how to master the MCAT through basketball metaphors.

No but seriously thanks lol, it just pissed me off that I got such an easy question wrong, especially an ORGANIC chem question ughh.

I know what you mean. That same question kept me from getting a 15 on the BS.

 

[Neutral]

I think question 20 on PS is a bit unfair;

The questions reads: A gas that occupies 10 L at 1 atm and 25 C will occupy what volume at 500 atm and 25 C.

The correct answer is b, Somewhat more than .02 L because of the space occupied by the individual gas molecules. I really wanted to put this answer, but I changed it to A (exactly .02L) after I read choice C: Somewhat more than .02 L because of the repulsions between gas molecules.

If we are treating the gas as a real gas that does not behave ideally, then B is certainly correct - but so is C. If we are treating it non ideally then the repulsions between molecules is present, so we would treat it the same as how the molecules would have volume.

Since I saw these as two correct answers, I eliminated them and chose A, as it represented the gas as an ideal gas.

Am i missing something? I really feel like b and c are equally correct answers.

 

[Vurday]

I think question 20 on PS is a bit unfair;

The questions reads: A gas that occupies 10 L at 1 atm and 25 C will occupy what volume at 500 atm and 25 C.

The correct answer is b, Somewhat more than .02 L because of the space occupied by the individual gas molecules. I really wanted to put this answer, but I changed it to A (exactly .02L) after I read choice C: Somewhat more than .02 L because of the repulsions between gas molecules.

If we are treating the gas as a real gas that does not behave ideally, then B is certainly correct - but so is C. If we are treating it non ideally then the repulsions between molecules is present, so we would treat it the same as how the molecules would have volume.

Since I saw these as two correct answers, I eliminated them and chose A, as it represented the gas as an ideal gas.

Am i missing something? I really feel like b and c are equally correct answers.

I also picked C on this question and realized immediately after the section that it would be B. I'm not sure if my reasoning behind this logic is correct, but the way I thought about it was that since the gas is under extremely high pressures, the applied force that it experiences is probably enough to make intermolecular repulsion negligible (think of squishing two like ends of a magnet together: if you add enough force, eventually they'll stay touching even though there's a repulsive force between them).

At that point, the reason that it'll be greater than 0.02L would have to be because the molecules take up some space as opposed to no space as the ideal gas theory states.

 

[Neutral]

I also picked C on this question and realized immediately after the section that it would be B. I'm not sure if my reasoning behind this logic is correct, but the way I thought about it was that since the gas is under extremely high pressures, the applied force that it experiences is probably enough to make intermolecular repulsion negligible (think of squishing two like ends of a magnet together: if you add enough force, eventually they'll stay touching even though there's a repulsive force between them).

At that point, the reason that it'll be greater than 0.02L would have to be because the molecules take up some space as opposed to no space as the ideal gas theory states.

That is a good point - though if the pressure is extremely high, wouldn't it make the intermolecular forces just as negligible as the volume? Or at 500 atm are the molecules literally as close together as possible? That can't be though, since the substance is still a gas, so there is ample space in between the molecules.

 

[Synapsis]

Synapsis: teaches troubled,high-risk urban youth how to master the MCAT through basketball metaphors.

No but seriously thanks lol, it just pissed me off that I got such an easy question wrong, especially an ORGANIC chem question ughh.

So easy to make silly mistakes. But I sure the adrenaline will help on test day lol.

 

[gettheleadout]

That is a good point - though if the pressure is extremely high, wouldn't it make the intermolecular forces just as negligible as the volume? Or at 500 atm are the molecules literally as close together as possible? That can't be though, since the substance is still a gas, so there is ample space in between the molecules.

It's just a hard question. As I understand it, the reason the real volume is greater than ideal at very high pressures is due to the significance of molecular volume specifically. Seeing your objection though, I have to agree the question is sketchy. The reason the molecular volume is significant is because it places a limit on gas compressibility, which occurs only because molecules repel each other. I believe answer C was intended, however, to be referring to the effects of non-negligible IMF on gas behavior, which, at moderate pressures or low temperatures, reduces gas volume below ideal. As the pressure gets very high (like 500 atm), this net attractive force is overcome by the fact that non-zero volume gas molecules can only be compressed linearly with pressure to a certain point, beyond which they take up enough of the remaining space that they begin to compress less.

But as you said, if they didn't repel each other then they could pack more closely and continue to compress linearly. Maybe the question is assuming that the IMF from London dispersion forces is only characterized as net attractive, so the exclusion of nearby molecules during high pressure compression is simply a matter of other molecules taking up space.

Honestly I think real gases, along with thermodynamics, is one of the most complicated content areas, and I haven't seen it explained to my satisfaction in anything I've read (two textbooks, three MCAT prep books, and several websites).