Cracking the SAT Chemistry Subject Test

(Marvins-Underground-K-12) #1
pH  =   pKa +   log 

Unfortunately, people often remember this equation incorrectly, either because
they’re used to taking −logs in acid-base chemistry or because they’re confused
about whether [A−] or [HA] is in the numerator. So here is a test to check to
make sure that the Henderson-Hasselbalch equation you jot down on test day is
correct.


Adding a base to any solution, whether it is buffered or not, always increases the
pH. The difference between a buffered and a normal solution is the size of the
pH change. If we increase [A−] in our Henderson-Hasselbalch equation, then pH
ought to increase.


pH  =   p   Ka  +   log 

Increasing [A−] makes the fraction bigger. Increasing a number also


increases that number’s logarithm, so + is also bigger, and the pH
has increased. We can see then that this is the correct equation. Adding a base
increases the pH.


Of course, the Henderson-Hasselbalch equation can be worked out for pOH, as
illustrated below.

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