A weak acid with pKa 4.76 is in a buffer containing ten times as much conjugate base as acid. What is the pH?
Answer and explanation
Correct answer: B. 5.76, from pH = pKa + log([A-]/[HA]) = 4.76 + log 10 = 4.76 + 1
- Step 1: Henderson-Hasselbalch: pH = pKa + log([A-]/[HA]). Step 2. log(10) = 1, so pH = 4.76 + 1 = 5.76. Answer: 5.76. Why not the others: inverting the ratio gives 3.76, forgetting the log gives a wrong figure, and pH equals pKa only for equal amounts.
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