At half-equivalence, the pH equals:

• A. pKa.
• B. 7.
• C. 14 - pKa.
• D. The pH of the added base.

Answer: (A)

At half-equivalence, the amounts of the weak acid and its conjugate base are equal, forming a buffer. The Henderson–Hasselbalch equation says pH = pKa + log([A−]/[HA]); with equal concentrations, the log term is zero, so pH equals pKa. This is why the pH matches the acid’s pKa at this point. The other possibilities don’t apply here: pH = 7 would only occur for a neutral solution or a strong acid–strong base titration at equivalence, not a weak acid titration; 14 − pKa isn’t a standard relation for pH in this context; and the pH isn’t simply the pH of the added base but is governed by the buffer ratio at half-equivalence.