2 BS AAMC #11 Questions

[bdc142]

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Hi all,
I had two questions about...two questions in AAMC 11

Item 123:
In TLC using silica gel plates, the analytes can interact with silica gel through hydrogen bonding. Which would most likely have the strongest interactions with silica gel?

A. Alcohols
B. Carboxylic acids

So I said B because carboxlyic acids have both a ketone and an alcohol group aka 2 places where you can H-bond. The AAMC answer says it's carboxylic acids because they are more acidic than alcohols and therefore their O-H groups are "capable of entering into more effective hydrogen bonding with silica gel than are the O-H protons in alcohols".

Can anyone clarify their explanation? I don't get how acidity affects hydrogen bonding. Also, is my reasoning incorrect?

Item 140 (passage about lung protection)
The walls of the alveoli are most likely composed of which of the following?
A. A single layer of epithelial cells
D. A single layer of epithelial cells with an underlying layer of dense connective tissue

I said D because the single layer allows diffiusion of gases, but the connective tissue (aka blood) brings it to the heart. The answer is A. I'm confused!

Thanks for any responses 🙂

 

[411309]

Stronger acids readily give up protons easier. The carbonyl group is electron withdrawing increasing the acidity making the molecule more acidic overall so it can more easily donate the proton for a bond I believe.

For the second one, I don't recall. But I assume dense connective tissue could interfere with gas exchange making it more difficult for the alveoli. Though im not positive on this question.

 

[CodeRedDew]

Stronger acids readily give up protons easier. The carbonyl group is electron withdrawing increasing the acidity making the molecule more acidic overall so it can more easily donate the proton for a bond I believe.

You don't need to have "proton-donation" occur for hydrogen bonding. Instead, the fact that the carboxylic acid contains a more acidic proton means that there is a larger dipole between the O-H bond in comparison to the alcohol's hydroxyl group (The O has a greater delta- and the H has a greater delta+). This could account for the fact that it makes it a better hydrogen bonding compound as a larger dipole equates to a slightly larger separation of charge, ultimately leading to a stronger hydrogen bond.

So I said B because carboxlyic acids have both a ketone and an alcohol group aka 2 places where you can H-bond.

However OP, your reasoning is not wrong and should have been sufficient enough to answer the question correctly.

In regards to the second question, I believe it is strictly referring to the walls of the actual alveoli and since blood is not a part of the actual alveoli then it makes choice D incorrect.