AAMC Sample Chem/Phys Questions-Pt-1

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svpatel
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Joined: Wed Jan 15, 2020 8:07 pm

AAMC Sample Chem/Phys Questions-Pt-1

Post by svpatel » Fri Jan 24, 2020 12:53 pm

2. Do lone pairs count in determine chiral centers? I found online that they do (eg for amines). I am not sure of this question. It looks like they don’t(bc only 5 chiral centers and 2^5 to get stereocenters) and I don’t know why.

6. SO I know myelin sheat doesn’t increase the capacitance of the axon. But then what does?

9. Idk how to do this problem. I tried but didn’t really get it. Do I use R= rho(L)/A and I = V/R?

11. The explanation says hydrogen ion concentration for this solution is greater than 1 × 10^–7 but much less than the concentration of the unknown compound itself. What is the conc of unknown compound? I don’t see it given in the passage. I did notice that it partially ionized so it must have had been acid, but I couldn’t figure out the strength, and got the ans wrong. What can I do to make this better next time?

16. I don’t really get the explanation. What does it mean by mutarotation and what does it have to do with ring-opening and hemiacetals?
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

Re: AAMC Sample Chem/Phys Questions-Pt-1

Post by NS_Tutor_Mathias » Fri Jan 24, 2020 3:37 pm

2. No, not really. This might be true for a carbide, but that seems like an edge case far beyond MCAT relevancy. I want to commend you though, this is a really good question conceptually, and the only reason it doesn't turn out the way you would expect (since we are after all talking about 4 different groups attached to a central atom) has a fairly obscure reason: https://chemistry.stackexchange.com/que ... al-centres

6. Decreasing the thickness of the plasma membrane, increasing the surface area or increasing the permittivity of the membrane (this one would easily lead to a breakdown of the charge separation). A myelin sheath effectively decreases the capacitance of the axon in the myelinated sections.

9. You are going to hate this answer.
It's just V = IR ---> I = V/R
and then realizing that to determine R we need the resistance per unit length of an axon times length of the axon.


11. We don't actually know and the explanation is a little inconsistent here. They do not give us any information to determine a concentration of this compound in the sample of water. All the evidence we have is individually a little sketch, but here goes:
- Weakly conducting electricity suggests incomplete dissociation (or low amount of sample relative to water)
- The passage strong suggests that each of these compounds is either a weak base or a weak acid. Anything that is likely to dissociate incompletely (and have a measurable pKa as a result) is a weak acid or base. It is also stated that these are buffering agents, which limits you to weak acids and bases as well. Finally, it would be okay to just extrapolate from what you know of compounds with the given formula and take it from there.

This particular question is a little frustrating because we really aren't given sufficient quantitative information about how much solute in how much water is being used for this experiment.

16. Mutarotation is the interconversion of anomers, most commonly in cyclic sugars.
mutarotation.png
(45.88 KiB) Not downloaded yet
(right-click, save as, then open in an image viewer. This is a file I just had ready on my drive and unfortunately still has transparency included)

You will have to review the mechanism by which this happens a little bit, but it does involve opening the ring first and then another intramolecular nucleophilic attack to close it back up. At its heart, the whole process happens because of the trigonal planar nature of the carbonyl carbon being converted to an sp3 carbon on cyclization, and the orientation of the newly formed bond being essentially just by chance.
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