AMCAS exam II question

[jmu29]

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Chem/phys question: question asks to calculate ph using Henderson hasslebach equation. In my UG class notes and online the equation shows log (acid)/(base) but in the answer description is shows log (base)/(acid). Any explanation? or can it be an AAMC error?

 

[JimKimSlim]

That is indeed the correct equation and the answer. Perhaps your class note had minus instead of plus on the equation. If you’re not sure, you can always derive the equation yourself. I’m happy to do so myself, if you’re not familiar with how to derive it.

 

[PlsLetMeIn21]

Their answer is right. I'd suggest that you try to reason it out when in doubt. The mixture in their question has more acid than base, and acid makes the pH lower, so it must be less than the pKa (< 6.37). The ratio is 10:1 and log of 10 is 1, so it's 1 lower than the pKa (= 5.37).