AAMC Sample Test - Chem/Phys Section

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Joined: Thu Jan 09, 2020 8:28 pm

AAMC Sample Test - Chem/Phys Section

Post by bbiebelberg » Thu May 07, 2020 10:49 pm

Hello! I have a few questions from the Sample Test C/P section, which I figured I'd group together. Sorry for the several questions but hopefully they are not too hard to answer!

Thank you!

Question 11
I chose D, strong acid, because of the statement that the compound *completely dissolved* in water. However, the explanation seems to draw a distinction between dissolution of the compound and ionization of H+. Could you elaborate on this? I was just relying on the factoid that strong acids/bases dissolve completely.

Question 22
I identified this as a redox reaction, but I had narrowed the choices to Redox and Ionization. Is there a good rule of thumb for determining whether a reaction is Redox vs ionization?

Question 38
I was quite confused about how to identify and follow the relevant carbon(s) through the mechanism, so I had no idea which answer would be right. Are you able to help explain the approach to this question and how to arrive at the right answer?

Question 46
I’m not familiar with this application of Archimedes’ Principle and didn’t follow the explanation. Would you be able to say a bit more about the derivation?

Question 47
Pantothenate — I understand that COOH will be deprotonated at pH=7 since COOH has a pKa below 7. But is there a good way to tell whether other functional groups would be affected? For example, how can we know the N would not be protonated and carry a positive charge?

Phosphopantothenate — here are we just expected to know that the phosphate group has pKas below 7?

Question 52
I’m good with E = hf, but were we supposed to calculate an energy from 140 keV? I’m actually not familiar with the eV unit so can you explain what an electron volt is and how to convert it to a value for energy?

Lastly, a just a general question: in a titration, what is the difference between the equivalence point and the endpoint?
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Joined: Sat Mar 30, 2019 8:39 pm

Re: AAMC Sample Test - Chem/Phys Section

Post by NS_Tutor_Mathias » Fri May 08, 2020 4:25 am

38 I will do later, let me plug a tablet in to more easily make a drawing for you :)

You kind of answered this yourself. Dissolution is not the same as dissociation. Solid sucrose dissolves in water. It does not however dissociate quite as easily. Same here: The protons of this weak acid did not all dissociate, but the entire thing did dissolve - meaning it remained in solution, not a precipitate. An additional hint here is that the solution conducts electricity only weakly. While small amounts of strong acids would have the same effect, the passage didn't specify that only an extremely small amount was used. In addition, the hydrogen ion concentration in solution would be equivalent to the amount of strong acid added divided by the volume of solution.

Easy: Assign oxidation states. Any reaction in which oxidation states change is a redox reaction, anyone who tells you otherwise is a liar.

In this case, it would suffice to look at Magnesium going from 0 to +2. Hydrogen additionally goes from +1 to 0.

Generally you should know the pKas for alcohols and amines, within a general range. It is often even enough to know that these values are plenty high (12-14+). That is really all. Conversely the threshold to protonate a secondary amide is too acidic to really care about here - and when this does happen it is on O, not the N. This is a depth of OChem unlikely to be on the MCAT, and you are fine with just "amides = unreactive".

Remember that a volt is a J/C. And an electron is just a fundamental unit of charge (given in the problem as 1.6 x 10^-19 C) So an electronvolt must just be a strange unit of energy (because C*J/C=J).

We can now just do this conversion, because eV * e = J (again: J/C * C). This yields 1.6 * 10-^19 J for one eV

We have 1.4*10^5 eV, so this in turn yields ~2.24 ^10-14 J for the energy of these gamma photons. Divide that by h (~22.4*10-15/6*10^-34 = ~3 * 10^-19 ). As usual for the MCAT, we only need to land within the correct power of 10 and we are good to go!

Endpoints are where indicators change color and rarely come up as separate concepts on the MCAT. Equivalence points are where, if adjusted for polyprotic/polybasic, moles titrant = moles analyte.
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