Kaplan FL 9 Question 25 (Acids and Bases)

[K1N6]

So I hit a question and i didn't quite know how to deal with it, nor did the explanation given when reviewing the exam give much insight.

The question is #25 on Kaplan FL9,

I've attached a print screen of the question and the explanation.

The basis for the answer i chose was just a guess and i moved on. I think in retrospect that after we get the Ka's that we must solve for something common to both sides of the equation. Since Bicarbonate is being used up as a reactant as it's being produced, it cannot be equal in both equations. This leaves H+, which i guess is a product of both reactions and would accumulate in the solution and reflect the pH. Thus, that would be the reason why we use that as a basis to set the two equations equal.

Am i on the right track here?

Much thanks,
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[K1N6]

Oh dang, follow up, i foolishly forgot to mention the two reactions needed to actually solve the problem!

 

[K1N6]

The question with explanation as well...

 

[kraskadva]

With any polyprotic acid, the loss of H+ beyond the initial dissociation makes very little (read negligible) difference to the pH (read [H+]) since it happens much less than the initial dissociation.

So, with that in mind, you can eliminate answers 1 & 2, because for both the first term would be much greater (since the [HCO3-] will always be much greater than the [CO3--])
Answer 3 can be eliminated, since the Ka's are switched, which is nonsense.
This leaves Answer 4, which is also what you'd get if you solved it through (but who wants to waste that time on the MCAT?).

 

[xtra333]

K1N6, I want you to first write out the two acid base dissociation equaitons. then, decide (or manipulate) them to get two equivalent expressions

 

[K1N6]

Thanks for setting me straight guys