## 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 pKa of 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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