AAMC CBT5 and 5R OFFICIAL Q&A

[Vihsadas]

This is the official Q&A thread for AAMC CBT5 and 5R.

Please post ONLY questions pertaining to AAMC CBT5 and 5R.
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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[notahappycamper]

I don't understand #5
According to table 1, what is the delta S of the following reaction?
2O3 --> 3 O2
S of O3 is 238.8 (J/mol*K)
S of O2 is 205.0 (J/mol*K)
[3*S of O2]- [2*S of O3]=delta S rxn
the answer is 137.4 J/(mol*K)
Why is it that the final units are in J/(mol*K) as opposed to J/K, because shouldn't the molar coefficients cancel out the moles in the denominator of the entropy expression?
Thanks

 

[notahappycamper]

?

 

[justadream]

Why do you add the acid first? Doesn't adding the acid protonate the attacking COO- (making it a COOH) and make that a worse nucleophile?

 

[Oh_Gee]

Why do you add the acid first? Doesn't adding the acid protonate the attacking COO- (making it a COOH) and make that a worse nucleophile?

it looks like the carbonyl being attacked becomes a better electrophile because it's O became protonated?

 

[justadream]

it looks like the carbonyl being attacked becomes a better electrophile because it's O became protonated?

Right the carbonyl being attacked becomes a better electrophile. However, the nucleophile attacking it becomes a weaker nucleophile.

So I guess being a better electrophile is more important than being a strong nucleophile...

 

[anisidi1985]

#26 in physical sciences - says Cu is oxidized (got that right) and Ag+ is reduced - I put Ag. Why is it Ag+? How are you to know whether it's ions present or not? I feel like this is a really stupid question but I just don't get it.

 

[anisidi1985]

Right the carbonyl being attacked becomes a better electrophile. However, the nucleophile attacking it becomes a weaker nucleophile.

So I guess being a better electrophile is more important than being a strong nucleophile...

also confused about this - i put heating then acidification; why does heating come after?

 

[DrDreams]

#26 in physical sciences - says Cu is oxidized (got that right) and Ag+ is reduced - I put Ag. Why is it Ag+? How are you to know whether it's ions present or not? I feel like this is a really stupid question but I just don't get it.

Ag is already in its reduced form, Ag+ can be reduced is how I thought of it. But I hastily answered the same way and now have smacked myself in the face.

Also, I was having a hard time with PS #43. I just realized how I should have thought of it. I calculated everything correctly but I just couldn't see it.

 

[DrDreams]

Why do you add the acid first? Doesn't adding the acid protonate the attacking COO- (making it a COOH) and make that a worse nucleophile?

In my notes, I have that dehydration of an alcohol requires hot acid. I wouldn't want to pour hot acid, so I'd rather add acid then heat it. That was my reasoning.

 

[pacer]

Can someone go over AAMC 5R question 121?

The correct answer states that no examples of tools were given in the passage but the last paragraph mentions choppers, scrappers and flakes.

 

[justadream]

Can someone go over AAMC 5R question 121?

The correct answer states that no examples of tools were given in the passage but the last paragraph mentions choppers, scrappers and flakes.

You need to post the question, especially since many people don't have the R versions of the test.

 

[young_gohard]

Can someone go over AAMC 5R question 121?

The correct answer states that no examples of tools were given in the passage but the last paragraph mentions choppers, scrappers and flakes.

Little late, but I got this question (86 on the CBT) wrong for the same reason and when I went back and read the passage, I realized the last paragraph abruptly stopped talking about Neandertals and started talking about tools in general. Then, it quickly loops back to Neandertals with these statements: "The Levallois technique was developed, enabling toolmakers to produce several large flakes from a single lump of rock. When the Neandertals came onto the scene, they further refined this technique." So the choppers and scrapers mentioned earlier don't apply to the Neandertals.

However, while AAMC asserts that NO examples of tools manufactured by Neandertals were given, clearly the passage says they made flakes. If the question had said "inventing artifacts" then I could see how their answer is right, but it says "manufacturing artifacts".

Maybe they're not aware of the definition of manufacture, though (the definition includes to "make" and also to "invent"):

to make or produce by hand or machinery, especially on a large scale.
to invent fictitiously; fabricate; concoct:

Question:
The author claims that the Neandertals were resourceful in manufacturing artifacts. The support for this conclusion is:

Answer: weak; no examples of their tools are given.
Answer I chose: weak; their tools remained few and crude.

 

[young_gohard]

Did anyone else notice how question 130 contradicts one of AAMC's explanations from a previous question in their Official Guide?

Question 130: If a person's gallbladder is removed, the person should restrict the consumption of:
Answer: triglycerides

However, in the explanation to bio question 27 in the guide, AAMC states: "Surgery to remove the gallbladder is fairly common, and most people do not have to follow fat-restricted diets after recovering from it."

 

[ahisma]

What is up with FL 5's verbal? You can miss 9 questions but still get a 15? Am I seeing this right or is it an error?

 

[Cawolf]

I replied in the test thread, but there are only 40 verbal questions. I think that should say 39-40.

 

[ahisma]

Did anyone else notice how question 130 contradicts one of AAMC's explanations from a previous question in their Official Guide?

Question 130: If a person's gallbladder is removed, the person should restrict the consumption of:
Answer: triglycerides

However, in the explanation to bio question 27 in the guide, AAMC states: "Surgery to remove the gallbladder is fairly common, and most people do not have to follow fat-restricted diets after recovering from it."

Well what other answer could it be for this question though? Can't overanalyze, just have to use process of elimination and move on.

But yes, this is the problem with including medicine on a multiple choice test for those with no medical training. Requires so much oversimplification it's sometimes wrong.

I also noticed they asked discrete questions about topics not on their official outline, like IUPAC nomenclature and aromaticity. I wonder if these are just hold overs from an older test or whether the AAMC official outline is actually a red herring.

 

[Cawolf]

@ahisma Do you mean the question about naming the imine?

If so, naming is definitely on the outline. Every Orgo subtopic has the very first item as "nomenclature".

 

[ahisma]

You're absolutely right. I was looking at mcat-review.org and saw IUPAC nomenclature under the "old topics." But I guess putting nomenclature under each subtopic pretty much covers the same thing.

I was specifically thinking about #117 the aniline and benzylamine question.

 

[Cawolf]

@ahisma

You don't need to know what they look like to answer that question.

They have the structures listed in the chart in the passage.

 

[Fetch22]

I have a question about number 108 in the BS section:
"When an initially heterozygous macronucleus undergoes repeated binary fission, the result will be"
I'm confused on how they got their answer. The question states the Tetrahymena is undergoing binary fission but then the answer starts discussing amitotic division. How can something undergo binary fission where the daughter cells are supposed to be identical to the parent but then also have uneven distribution of alleles because of amitotic division? What am I missing?

Also, I posted this question in a separate thread but later realized this would be a better place to post my question. Would someone mind telling me how to delete that other thread?

 

[Cawolf]

@Fetch22 You cannot delete it, you would need to ask a moderator. They would likely just close it.

In regards to your question, I also thought the passage was challenging.

This was not normal fission, these macronuclei were 45-ploid and the passage stated each daughter cell received "an uneven amount of DNA from the amitotic macronucleus".

This was enough information to eliminate the other choices. Out of the context of the passage I would agree with you, but this was not a discrete style question.

 

[ahisma]

@ahisma You don't need to know what they look like to answer that question. They have the structures listed in the chart in the passage.

Yup but I forgot degree of substitution for amines doesn't inherit the attached carbon's degree of substitution like for alcohols or halogens. Oh well, now it's burned into my memory.

 

[young_gohard]

Well what other answer could it be for this question though? Can't overanalyze, just have to use process of elimination and move on.

But yes, this is the problem with including medicine on a multiple choice test for those with no medical training. Requires so much oversimplification it's sometimes wrong.

I also noticed they asked discrete questions about topics not on their official outline, like IUPAC nomenclature and aromaticity. I wonder if these are just hold overs from an older test or whether the AAMC official outline is actually a red herring.

Yeah, it's the only possibility on that question and I'd gladly take a whole test of simple but contradictory questions like that. But it's somehow soothing (and simultaneously worrying) that AAMC makes mistakes too. Also agree about apparent the topic discrepancies. I can only imagine how many hours I've spent going over topics that AAMC doesn't intend to test us on.

 

[mslm5401]

Did anyone else have a tough time with the Picasso passage? I got more wrong in that one passage than the rest of the verbal combined. And if someone does understand it, would you please explain #82?

The last paragraph describes how Picasso said that the portrait of Stein will look like her if the viewer changes his/her perspective–isn't this an example of creating something with multiple layers of meaning? I still don't understand why that's the right answer, let alone why some of the other ones are wrong. Thanks in advance!

 

[ahisma]

Yes, I also found that passage difficult as did at least a handful of other posters here. I suppose the MCAT authors believe writing a novel with "many layers of meaning" requires more "understanding" than writing a novel with "terse realistic tone." Seems highly subjective to me and I have no idea why they think they can test on such things. Verbal questions like these feel more like "guess what the MCAT authors think."

 

[mslm5401]

Yeah wait, that actually kinda makes sense. So like which option needs understanding rather than just raw feeling... yo thanks

 

[ahisma]

So think, "which answer would a robot pick?"

 

[mslm5401]

^well, that was my line of thinking, which is why i picked terse and realistic. I think your first explanation was more clear, in that (B) has to be emotional, and (c) and (d) could apply to either, so only (A) is left.

 

[ieatshrimp24]

Can anyone explain #24? I don't understand how to predict what gases evolve during an oxidation-reduction reaction. So if anyone can explain that to me as well as how to answer #24 correctly, it would be much appreciated. Thanks.

 

[mslm5401]

@ieatshrimp24 You know that copper is going to lose electrons (i.e. be oxidized). So the question is asking what's going to be reduced (i.e. gain electrons lost by Cu). You're left with the option of the H+ in solution, or the NO3- . It is possible that some H+ is reduced to form H2, but the nitrogen in NO3- wants those electrons so much more. Nitric acid is a well known, very powerful oxidizing agent, so that will grab the electrons and let go of the oxygens its bonded to. This causes NO gas to evolve. The reaction should look something like this:
3Cu(s) + 8HNO3(aq) ——> 3Cu(NO3)2(aq) + 2NO(g) + 4H2O(l)

As a side note, generally, Cu in a solution of HNO3 should technically produce nitrogen dioxide, NO2, which tripped me up for a little bit. But I'm assuming that the copper strip was placed in the nitric acid and at least a teeny bit of water, because then the nitric acid can dissociate into its components and react from there.

 

[ahisma]

Yes I think #24 is a little tricky. You have to know that Cu will need to be oxidized for a reaction to take place, as is the trend with transition metals giving away their electrons to be stable. The question stem says there's no reaction with Cu and H2O. This means H+ is not reduced to yield H2(g). Something else will be reduced, either N or O. So you have to write a reaction where 1. Cu is oxidized and 2. either N or O is reduced. Thus the most likely reaction is:

NO3- + Cu --> CuO2 + NO (Cu oxidized, N reduced)

The other answers do not result in N or O being reduced with Cu being oxidized so can be eliminated.

 

[plage noire]

@ieatshrimp24
As a side note, generally, Cu in a solution of HNO3 should technically produce nitrogen dioxide, NO2, which tripped me up for a little bit. But I'm assuming that the copper strip was placed in the nitric acid and at least a teeny bit of water, because then the nitric acid can dissociate into its components and react from there.

I'm not positive, but I believe that NO2 often decomposes to NO and O2 (2NO2 --> 2NO + O2) so that's how NO would be formed even though NO2 is technically the product. Pretty sure I came across that in gen chem from time to time...

The question stem says there's no reaction with Cu and H2O. This means H+ is not reduced to yield H2(g). Something else will be reduced, either N or O. So you have to write a reaction where 1. Cu is oxidized and 2. either N or O is reduced.

Perfect explanation, this question tripped me up until reading this. Thanks!

 

[ahisma]

Glad it helped. It confused me too at first.

NO2 only decomposes to NO and O2 under high temps. Under standard conditions formation of NO2 is favored (page 1 here and then example 19-5 for ΔG).

For a galvanic cell, the species with the highest reduction potential will be reduced. According to this chart, NO(g) has a higher reduction potential than NO2(g) so I believe that's theoretically why NO forms instead of NO2.

 

[mslm5401]

I'm fairly certain NO only forms from reduction in the presence of H2O. This is what I had learned in gen chem (my tests had fairly in depth problems like this)
http://www.angelo.edu/faculty/kboudrea/demos/copper_HNO3/Cu_HNO3.htm

 

[ahisma]

Doesn't an aqueous solution mean water is present? Another perspective here: http://chemistry.stackexchange.com/...c-acid-to-nitric-oxide-and-concentrated-nitri

I'm glad this is outside the scope of the MCAT.

 

[xbg5567]

Can someone explain why Toluene can't form H-bonds with water? Is it because none of the hydrogens in toluene are partially positive?

 

[Styrene]

Can someone explain why Toluene can't form H-bonds with water? Is it because none of the hydrogens in toluene are partially positive?

Hydrogen bond donors have a hydrogen atom bound to one of nitrogen or oxygen; hydrogen bond acceptors have at least one electronegative atom with a partial negative charge. None of these is in toluene.

 

[ahisma]

Hydrogen bond donors have a hydrogen atom bound to one of nitrogen or oxygen

Fluorine as well.

 

[ml200201]

Choice A is true, but Choice B is better because it more directly answers the question, in regards to the arthropod/insect/choelom question