AAMC Practice Test 5 PS Passage 2

[nindra]

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My overall score was pretty satisfactory (35), but I hugely underperformed in the PS section given that it has always been my strength (almost always in 12-14 range). As I reviewed the incorrect answers, I noticed most were simple, lazy mistakes. But there was one problem that even after looking at the AAMC explanation I didn't understand the logic.

Its number 14. This is the chemical reaction:

n(CDP) <> (CP)n + n(HPO4)2-

(Note: n on CP is a subscript)

Question: With respect to (HPO4)2-, what is the concentration of the polymer NP?

The answer is: 1/n (HPO4)2-....by why? I thought their concentration had a direct correlation because an increase in n would increase both given that in one its a coefficient and in the other its a subscript.

Thank you.

 

[hiaips]

My overall score was pretty satisfactory (35), but I hugely underperformed in the PS section given that it has always been my strength (almost always in 12-14 range). As I reviewed the incorrect answers, I noticed most were simple, lazy mistakes. But there was one problem that even after looking at the AAMC explanation I didn't understand the logic.

Its number 14. This is the chemical reaction:

n(CDP) <> (CP)n + n(HPO4)2-

(Note: n on CP is a subscript)

Question: With respect to (HPO4)2-, what is the concentration of the polymer NP?

The answer is: 1/n (HPO4)2-....by why? I thought their concentration had a direct correlation because an increase in n would increase both given that in one its a coefficient and in the other its a subscript.

Thank you.

I not 100% sure that I understand what you're trying to say, but I'll take a stab at it.

An increase in n still produces only one unit / equivalent of polymer. (The polymer produced is longer, but the *amount* of polymer produced, in terms of moles or equivalents, is independent of n). Since only one unit of (CP)n is produced per n units of (HPO4)2- (according to reaction stoichiometry), the ratio of product concentrations must be 1:n.

 

[nindra]

Hey hiaips,
Thanks for your response. I think you understood my question right even though I wasn't very clear. It all makes sense now...I totally forgot that its the actual molecule that becomes longer, not the number of molecules.

Thanks again. Really appreciate it.