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AAMC chemistry question pack Q # 24
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Sophia_6535
#1 Posted : Tuesday, May 18, 2021 10:02:49 PM
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In the last question of passage 4, I don't understand how they got the ratio 100:1 from
10^2 = [HPO4^-2]/[H2PO4^-]

Can someone please help me out? Thanks!
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INSTR_Katrina_128
#2 Posted : Tuesday, May 18, 2021 10:56:24 PM
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Hi Sophia!

Great question.

The ions HPO4^-2 and H2PO4^- differ from each other by ONE H+ ion, so they are acid/base conjugate pairs!
Since H2PO4^- HAS the additional H+, it is the acid, and HPO4^-2 is MISSING the H+, it is the conjugate base. So, [HPO4^-2]/[H2PO4^-] is [base]/[acid].

This question is a perfect example of the Henderson-Hasselbalch equation:

pH = pKa + log([base] / [acid])

In the passage, we are told pH = 8.7 and we are given pKa = 6.7 in the problem statement. We are being asked to solve for [base]/[acid], so I will replace this term with x in the equation to make it easier to write out the equation.

8.7 = 6.7 + log x

Then, solve for x.

log x = 2

To "get out of" a log, we need to do the opposite action, which is "10 to the power of" on both sides.

10^(log x) = 10^2

Therefore, x = 10^2 = 100.

Let me know if this helps. :)
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