alleryThe pKa of a weak acid HA is 4.75. Find pH of an equimolar mixture of HA and A⁻:
A4.75
B7
C5.25
D9.25
Answer & Solution
Correct answer: A. 4.75
**By Henderson-Hasselbalch:** pH = pKa + log([A⁻]/[HA]).
**At equimolar:** log(1) = 0, so pH = pKa = 4.75. This is the principle of a buffer at maximum capacity.
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