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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[dimopiwo]

208.
In the reaction described by Entry 4 of Table 1, a
1:1 mixture of two products was observed, and
the second product was identified as the
enantiomer of the product shown in the table.
What is the structure of the second product?

(basically the question is asking what is the enantiomer of the structure in entry 4 of the table that's attached below)

the table is attached.

I am wondering about the dashed carbonyl group. It is backwards - comparing to what we usually see: functional group would point away from the central atom or ring. here the ring is pointing away from the carbonyl group! wouldn't this change the dash to a wedge that points away from the ring and therefore changes the enantiomer for the correct answer?

 

[FeinMS]

To find an enantiomer of a compound, you need to invert all of the stereocenters of the compound (wedge to dash and dash to wedge)

 

[erweightlifter]

I cannot understand number 14 on the PS section for AAMC 5. If you have two products in a reaction, how do you determine their concentrations relative to one another? Makes absolute no sense to me. Anyone understand this question?

 

[dragoneye776]

I'll take a stab.

Background info: most of us probably thoughr of that Zinc and HCl reaction producing ZnCl2 and H2. It made sense to us back then, it's a redox reaction. Well HCl is a strong acid so it dissociates to H+ and Cl-. The Zn then reduces the H+ and that's how you end up with H2 gas and ZnCl2. Zn has the option of reducing H+ or Cl-, however notice that Chlorine already has a negative charge (it's got enough electrons).

In Cu and HNO3, the same thing happens, we have the acid dissociating into H+ and NO3-, with the Cu ready to reduce. Most people would say "H+" is reduced since it has a positive charge, BUT we need to look at the lewis structure for NO3-.

Notice how nitrogen has a positive charge in all the resonance structures? So now we're trying to decide who wants to be reduced more, N+ or H+? Think about the electronegativities of the two atoms. We KNOW that F, O, and N are more electronegative than H because these atoms can engage in hydrogen bonding. Electronegativity is the tendency is attract electrons to itself. So N+ seems to be the clear winner in terms of who will be getting the electrons. [Electronegativity is 3.04 for N and 2.20 for H]

The reaction with Zn and HCl is easier to deal with because we don't have two electron deficient atoms to choose between. However, what would you expect from Zn and HNO3?

What about the reaction between Zn and H2SO4 when there H2 is formed because the H+ is reduced instead of the S, even though S is more electronegative?

If you answer it's because S is not as electronegative as N, then why does Cu + H2SO4 create SO2 (and CuSO4 and H2O)?

 

[Rob Biomed]

AAMC CBT 5 question #38

The premise of the passage is that a child on a sled slides down a hill on a taboggan from point A (at the top of the hill) to point B (at the bottom of the hill). Between points A and B, the sled and rider are opposed by a constant 50N frictional force.

Question: Which of the following best describes the energy conversion that is taking place when the toboggan is sliding from point A to point B?

A) Kinetic to potential and thermal
B) Kinetic and potential to thermal
C) Potential to kinetic and thermal
D) Potential and kinetic to thermal

A and B are immediately crossed off, so it's between C and D.
I chose D, but the correct answer is C...I am kind of thrown off by the wording in this one. I get that the energy conversion is Potential to kinetic, but isn't the thermal energy a result of the kinetic energy?

I thought the answer should be Potential to Kinetic to thermal. How does energy go directly from potential to thermal? Maybe it's the wording of the problem, or maybe I'm just not quite seeing it...but can someone help me out on this and elaborate on why the answer is c?

Cheers

 

[FeinMS]

Friction causes some of the energy to be converted to heat. As it comes down, friction is doing work on it, so potential->thermal.

 

[gettheleadout]

Friction causes some of the energy to be converted to heat. As it comes down, friction is doing work on it, so potential->thermal.

Technical point but conversion of kinetic to thermal energy via friction involves work, not heat flow, since rubbing together two hands of equal temperature raises the temp of both. I know what you probably meant but "heat" can be a funny term.

 

[Temperature101]

AAMC CBT 5 question #38

The premise of the passage is that a child on a sled slides down a hill on a taboggan from point A (at the top of the hill) to point B (at the bottom of the hill). Between points A and B, the sled and rider are opposed by a constant 50N frictional force.

Question: Which of the following best describes the energy conversion that is taking place when the toboggan is sliding from point A to point B?

A) Kinetic to potential and thermal
B) Kinetic and potential to thermal
C) Potential to kinetic and thermal
D) Potential and kinetic to thermal

A and B are immediately crossed off, so it's between C and D.
I chose D, but the correct answer is C...I am kind of thrown off by the wording in this one. I get that the energy conversion is Potential to kinetic, but isn't the thermal energy a result of the kinetic energy?

I thought the answer should be Potential to Kinetic to thermal. How does energy go directly from potential to thermal? Maybe it's the wording of the problem, or maybe I'm just not quite seeing it...but can someone help me out on this and elaborate on why the answer is c?

Cheers

As the child is at the top of the hill, the velocity is zero. Therefore, the child has only potential energy (PE). Once the child sliding down, some of the PE is being converted KE, and some of KE will be lost due to friction... PE is being converted to KE and Thermal Energy.

 

[Summerlands]

For #38, couldn't you also have immediately eliminated answers B & D because they say the same thing?

 

[Heday]

What about the reaction between Zn and H2SO4 when there H2 is formed because the H+ is reduced instead of the S, even though S is more electronegative?

If you answer it's because S is not as electronegative as N, then why does Cu + H2SO4 create SO2 (and CuSO4 and H2O)?

Couple of things I want to address, although I probably won't be able to answer your question(s) because I don't know the answers well enough to do so.

1) It's not just electronegativity that you look at, but also the resonance structures.
2) The question you asked in regards to sulfuric acid, is it concentrated or dilute? It's my understanding that the concentration could product different results than you say.

ie: dilute acid + zinc could produce so2, while dilute acid + copper results in no reaction
concentrated acid + zine could produce h2 while concentrated acid + copper results in so2.

I'm not sure what is actually produced in what scenario, but is it possible you're changing more than one variable at a time?

 

[enervate]

Alright I've read through this entire thread, and while some people have addressed this question, I haven't seen too many answers that really flesh it out.

I'm referring to question 24, PS section and it is as follows:

"When a strip of Cu is placed into H2O (l), no change is observed. However, when a strip of Cu is placed into a solution of HNO3 (aq), a gas evolves. What is the most likely identity of the gas?"

I, like many others, chose H2. The correct answer was NO.

How does this problem work? Does Cu form a bond to HNO3? Or Does Cu simply lose electrons to become oxidized, while HNO3 picks them up to be reduced? So we would have Cu 2+ and... HNO32-? If nitrogen picks up those electrons it would have 8 from its bonds to oxygen, picking up another two would im guessing make one of those bonds go to a lone pair on the oxygen to make nitrogen have 3 sigma bonds to oxygen, a lone pair, and 2 O's with -1 charges?

I don't really know what I'm doing when it comes to this problem so any help would be greatly appreciated!

I just wanted to bump this because I also incorrectly answered H_2. If NO is evolved from HNO_3, where do the remaining two oxygen atoms go? Kaplan tried to explain that CuO_2 is formed (below) but there is apparently no such thing (except enzyme catalysis). The passage shows that Cu reacts with O to form Cu_2O or CuO. Is the remaining oxygen supposed to form a hydroxide molecule with the protons?

 

[JingleChips]

I guess I should add my input to this test here (i took the 5R).

PHYSICAL SCIENCES

The 1/n [HPO3] question got me, as well as the NO gas question. The wave question was ambiguous, but none the less I was working too quickly to even notice the ambiguity and got it wrong.
Has anyone figured out a plausible explanation for the NO gas question?

Also ran into an example where knowing information in too much detail can actually hurt you on the MCAT:

PS 76. (The principal quantum number is a measure of which of the following:?)

In my mind, I thought this was AAMC trying to be tricky - and since I knew that the principal quantum number does set the limit on the max # of valence electrons, I answered that it was the measure of the # of valence electrons that orbit the nucleus.

This is obviously wrong, as the answer choice didn't say "CAN orbit the nucleus" - it said "THAT orbit the nucleus" - implying that you could determine exactly how many valence electrons are orbiting the nucleus given just n=3, which is obviously not true.
(for those who are curious, maximum # of valence electrons = 2n^2).

VERBAL

wowowowow wtf. I've always been awful at verbal compared to my other sections, but this section was just weird and just shows how well AAMC estimates the relative difficulty of passages (at least in my case). I had two passages 100% correct, which I've never even had one perfect passage before. My score on the first 30 was 26/30. But then picasso... and more. Score on the last 30 was 14/30, getting 3/10 on picasso, 2/5 on neanderthals and 2/5 on the poem one. Still got the same verbal score as AAMC 3R and 4R though, so it just showed me how well AAMC can balance a hard passage with easier ones / curves.

BIO

I don't have much to comment here, missing about 1q per passage, there was no passage that was really killer - although I think I got lucky on the macro/micronucleus passage.

 

[ipmed]

I have attached a screenshot for the questions I am having difficulty with. Essentially, Acidification followed by heating and heating follwed by acidification with an acid anhydride: what is the difference?. Also Why do SN reactions start with adding the nucleophile and some start with pronating the carbonyl carbon

 

[ipmed]

I just wanted to bump this because I also incorrectly answered H_2. If NO is evolved from HNO_3, where do the remaining two oxygen atoms go? Kaplan tried to explain that CuO_2 is formed (below) but there is apparently no such thing (except enzyme catalysis). The passage shows that Cu reacts with O to form Cu_2O or CuO. Is the remaining oxygen supposed to form a hydroxide molecule with the protons?

How about the Cu + NO3 + H+ = CuO + NO + H2O. Its nitric acid, so there is equal H+ and NO3 in solution, and the H+ combines with the oxygen thereby reducing O2 in solution. I think that the point is to pay attention to the fact that you cant have H2, because WHAT IS BEING REDUCED TO GET IT.

 

[dudewheresmymd]

Hey everyone, for this question, I was going to select plasma membrane but chose not to as I thought it wasn't "an organelle" I get that the mutation of the AcH receptor would most likely present itself in the plasma membrane. Is this question poorly worded?

Thanks!

 

[dudewheresmymd]

Why the heck is C the right answer for this question? Isn't B equally valid? Anyone care to clarify?

 

[txlonghorn2314]

Delete.

 

[inasensegone]

I thought examkrackers guys did a pretty good job explaining the Copper & Nitric acid problem here.

 

[dudewheresmymd]

I thought examkrackers guys did a pretty good job explaining the Copper & Nitric acid problem here.

I actually found the answer a few days ago in my Barron's AP chem textbook and they said the same thing as the EK dudes, thanks so much @inasensegone !

 

[inasensegone]

I actually found the answer a few days ago in my Barron's AP chem textbook and they said the same thing as the EK dudes, thanks so much @inasensegone !

No worries! Hopefully future people can find use in the EK video.

That Picasso passage though.... so stupid.

 

[dudewheresmymd]

No worries! Hopefully future people can find use in the EK video.

That Picasso passage though.... so stupid.

haha picasso wasn't that bad, I think I missed 2 questions on that...but agreed very dense :/ I find that on the harder passages sometimes if you can hang on for dear life, the questions aren't that bad..

Also, any idea for the catalytic amount question "can produce much O2" item 6

Indeed, that ek video is pretty nifty! just watched their explanation for tetrahydrema...still don't understand why the answer is 25% though for the genetics question on that passage...the EK dudes just assumed it was humans and did a test cross lol wtf?

 

[inasensegone]

haha picasso wasn't that bad, I think I missed 2 questions on that...but agreed very dense :/ I find that on the harder passages sometimes if you can hang on for dear life, the questions aren't that bad..

Also, any idea for the catalytic amount question "can produce much O2" item 6

Indeed, that ek video is pretty nifty! just watched their explanation for tetrahydrema...still don't understand why the answer is 25% though for the genetics question on that passage...the EK dudes just assumed it was humans and did a test cross lol wtf?

I haven't reviewed my bio questions yet (just about to start)... but for item 6: You have to look at reactions 3-5. For rxn 3: 1 CFC (F3CCl) yields F3C radical and chlorine radical. This chlorine radical is shown as reacting with Ozone (O3) in rxn 4 and producing ClO radical and O2. This ClO radical then goes on to react with a lone Oxygen atom to form O2 and to regenerate the Cl radical (like the one that was used as a reactant in rxn 4.

In answers A & B it says that one equivalent of CFC is required to produce one and two equivalents of O2, respectively. The only way either one of these answers would be correct, is if the reaction had terminated somehow after the formation of one or two equivalents of O2. This is not what is shown in the reactions though. It shows that the radicals keep on propagating. (At the end of rxn 5, a total of 2 equivalents of O2 have been produced, but there still remains a Cl radical and this can go on to keep producing more O2)

In answer D, it says that the CFC is produced by a catalytic amount of O2, and this is exactly opposite of what is indicated by the reactions, therefore this can't be correct either.

 

[orangeblue]

.

 

[dudewheresmymd]

1) The answer explanation states that the macronucleus does not go through the process of meiosis. Is this answer specifically referring to the words "elimination of some micronuclear DNA sequences from the 1) caption above? Since the micronucleus is the germ line diploid which would undergo meiosis and therefore elimination of micronuclear DNA may involve meiosis?

2) Is this a psuedo discrete? How do we reason that rRNA genes are self-replicating?

3) The fact that the macronuclei do not participate in mating..is this derived from the passage statement that the micronuclei are germ line cells whereas the macronuclei is the site of gene expression and are not "gametic" cells? In humans, we only consider the mating of the 23 pairs of chromosomes in gametic/gamete cells, right?

Thanks!

 

[dudewheresmymd]

As the child is at the top of the hill, the velocity is zero. Therefore, the child has only potential energy (PE). Once the child sliding down, some of the PE is being converted KE, and some of KE will be lost due to friction... PE is being converted to KE and Thermal Energy.

how did you infer that the velocity is 0 at the top of the hill? I didn't see it directly stated in the passage, got the question right the first time, but was wondering about this today.

 

[benjaminl1nus]

how did you infer that the velocity is 0 at the top of the hill?

the assumption that all the energy is PE (mgh) means that KE ((mv^2)/2) must be 0, and therefore v=o. is that helpful??

 

[Gauss44]

Can anyone explain #85 and #86 in the Neanderthal passage? Ideally, I would like your opinion of why the right answers are right and the wrong answers wrong, as well as, how you came to the correct answer (or could have). Thanks!

(For those who's test is numbered differently, those are the 3rd and 4th questions in that passage.)

 

[sillyjoe]

I have scored a 14 on every bio section up until this exam. Passage III about the Tetrahymena destroyed me. For those that were successful with this passage, how did you attack it? What phrases in the passage helped you hone in with questions?

I found challenging inferring for certainty that the macronucleus does not get passed on. They hinted at it in the first paragraph, but solidified it in the schematic. However, during a timed exam I guess I missed the circles turning into squares etc. that gave that away.

Also in the question when they discuss the heterozygous macronucleus undergoing repeated binary fission... where in the paragraph can you assume that these are all going to be amitotic divisions?

Any advice on how to attack a passage like this would be much appreciated!

 

[Padfoot]

I didn't understand item 40: what is the formula for the compound tert-butylamine? It says that the nitrogen of ((CH3)3C)3NH has 9 electrons on it. How is this?

 

[Mushrooomboy]

CBT #5: Item 117 Ochem

Did anyone understand why heating was required in addition to acidification for this problem?

 

[jaybird12]

CBT #5: Item 117 Ochem

Did anyone understand why heating was required in addition to acidification for this problem?

(Two years later): Because anhydrides are more reactive than carboxylic acids. To reverse Equation 3 and go from carboxylic acids to anhydrides, we are going from a less reactive compound to a more reactive compound. To do that, you need heat.

 

[jaybird12]

BS #129. I answered C because I thought that in open circulatory systems, the contraction of skeletal muscles helps move the hemolymph around, which seems like an even more important selective advantage than A) increased activity. But the answers say that C is wrong because "The muscles of the circulatory system include the cardiac muscle of the heart and the smooth muscle of the blood vessels." But this is an open circulatory system, without blood vessels, although there is still a heart.
So I'm not convinced by the answer's explanation. Can anyone provide a better explanation as to why A is correct and C is incorrect?

 

[JAH360]

#23 PS
The explanation says "looking at the table we see that both Zn and Hg form compounds with sulfur of the formula MS..."

I think I might be blind because I see nothing in either of the tables, or in the entire passage, about Hg. Is there something glaringly obvious I'm missing?

 

[pepocho]

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[pepocho]

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[JAH360]

I dont have the exam in front of me but the tables on the exam (if I recall correctly) show Zn and Hg form a compound with another anion and the compound was something like HgX_2 and ZnX_2, this implie the anion has a -1 charge and that Zn has a +2 charge since there needs to be two anions to even out the charges

Yeah, the chart shows Zn, but it doesn't show Hg. I'm wondering if they changed it and just never changed the explanation?

 

[Mushrooomboy]

CBT: #128

Isn't longevity a good enough evidence for species long life spans?

 

[pepocho]

.

 

[pepocho]

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[JAH360]

hm, my bad I believe I was thinking of another question, I pulled the AAMC 5 in front of me, but I have an R version, can you type out the question because I cannot find it

It's in passage IV. The table in the passage makes no mention of Hg.

 

[pepocho]

.

 

[JAH360]

You were correct, they did mess up on their answer key. Tsk tsk, AAMC. Here is the explanation in my R exam
" Looking to the passage for a clue about the product that would most likely form between Cd and S, we notice in
Table 1 that Zn and S form the compound ZnS. Since Cd is in the same family as Zn, it is reasonable to expect that Cd and
S would form the compound CdS. Now sulfur, like oxygen, is most commonly in a –2 oxidation state in its compounds
with other atoms. If the oxidation state of S in CdS is –2, then the oxidation state of Cd must be +2"

I hope it helps

thanks!

 

[DocBella]

ggrrrrrrr!! it's so sad that instead of testing our KNOWLEDGE as future doctors, they're testing us on how well we can detect the test maker's trickery! so frustrated with missing simple questions that are designed to be traps!!!!

 

[BornToLead]

Passage 3, Qs. 20
To find the E why can't we use the equation E=h/wavelength?
Why does it has to be E=hc/f?

Please help me out with this one! Thanks

 

[deleted388502]

Passage 3, Qs. 20
To find the E why can't we use the equation E=h/wavelength?
Why does it has to be E=hc/f?

Please help me out with this one! Thanks

The planck-einstein relation is E=hv; the speed of light is part of this relationship because the 'velocity' that you substitute in for in this case is the speed of light relationship (c = v x wavelength).

 

[JAH360]

Passage 3, Qs. 20
To find the E why can't we use the equation E=h/wavelength?
Why does it has to be E=hc/f?

Please help me out with this one! Thanks

I think you meant to say E=hc/wavelength, because E=hc/f isn't the correct equation for E, which is generally written as E=hf. If I understand what you're asking, you're making a simple mistake that's very easy to make. Wavelength and frequency aren't direct reciprocals of one another. The inverse of frequency is period. Frequency is equal to the speed of light (c) divided by the wavelength (and vice-versa). Thus, because f = c/wavelength, then E=hf=hc/wavelength.

 

[BornToLead]

Yes you got me right, I made a mistake typing that.
And thanks for that explanation! I get it now!

 

[Oh_Gee]

can someone explain how the ozone reaction works. adding CF3Cl to the air increases O2 so if there is an abundance of O2, shouldn't more O3 be formed?

 

[Oh_Gee]

Maybe this example will help.

Ca(OH)2 <----> Ca2+ + 2OH-

For every 2 moles of OH- there is 1 mole of Ca2+. Therefore, for every 1 mole of OH- there are 1/2 moles of Ca2+.

Now just replace the coefficient of OH- (2) with n and you get your answer: ratio = 1/n : 1

sorry to bump this but is the (CP)n the OH- in your example? the wording "with respect to" confuses me

 

[Oh_Gee]

the passage never balanced the equation, so "n" just represents any integer.

its asking about the relationship between the polymer (CP) and HPO42-. the equation says for every mole of CP you have n mol of HPO42-.

you can just plug in numbers for n...in this case i will use the number 3. if you have 3 moles of HPO42- that means you will have 1 mole of CP. in other words, for every 1 mole of CP you will have 1/3 (1/n) amount of HPO42-.

did tha tmake sense? its kind hard communicate over SDN

if you have 1 mole of CP and 3 moles HPO4 2-, doesn't that mean you have 3 mols of HPO4 2- for every mole CP which means the answer is A?