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MCAT Question A Day - 4/8/14 - Answer!

A student wishes to prepare a buffer whose pH is 6.3, using an acid HA with a pKof 6.0. Which expression below will correctly express the ratio R of HA to NaA that must be mixed in order to achieve the desired pH?

A.  6.3 = 6.0 + log R
B.  6.0 = 6.3 + log R
C.  10-6.3 = 10-6.0 + log R
D.  6.3 = log 6.0 + R

The correct answer is (B). Rearrange this expression to get the familiar Henderson-Hasselbalch equation; the trick is that the H—H equation uses the ratio of conjugate base to conjugate acid, the reciprocal of R as defined here. So,

6.3 = 6.0 + log (1/R)
6.0 = 6.3 - log (1/R)
6.0 = 6.3 + log R

If you do not notice this, you’ll probably choose (A).

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