Why is the pH at the equivalence point typically greater than 7 in a weak acid/strong base titration?

Why is the pH at the equivalence point typically greater than 7 in a weak acid/strong base titration?

• A. The conjugate acid of the base dominates.
• B. The solution is still buffered at pH 7.
• C. The salt of the weak acid with strong base hydrolyzes to produce OH-.
• D. Water autoionization drives the pH toward 7.

Answer: (C)

At the equivalence point in a weak acid–strong base titration, all the weak acid has been converted into its conjugate base, which appears as the salt of the weak acid with the strong base in solution. This conjugate base reacts with water in a hydrolysis process: A− + H2O ⇌ HA + OH−. The production of hydroxide ions makes the solution basic, so the pH ends up greater than 7. The extent of this hydrolysis is determined by the base’s hydrolysis constant, which is Kw divided by the acid’s Ka; because the original acid is weak (Ka is small), the conjugate base is relatively effective at generating OH−, pushing the pH above neutral. The other ideas don’t fit because a conjugate acid would tend to acidify the solution, buffering at pH 7 isn’t how the equivalence point behaves in a weak-acid/strong-base titration, and water’s own autoionization isn’t the driving factor in this basic shift.

At the equivalence point in a weak acid–strong base titration, all the weak acid has been converted into its conjugate base, which appears as the salt of the weak acid with the strong base in solution. This conjugate base reacts with water in a hydrolysis process: A− + H2O ⇌ HA + OH−. The production of hydroxide ions makes the solution basic, so the pH ends up greater than 7. The extent of this hydrolysis is determined by the base’s hydrolysis constant, which is Kw divided by the acid’s Ka; because the original acid is weak (Ka is small), the conjugate base is relatively effective at generating OH−, pushing the pH above neutral. The other ideas don’t fit because a conjugate acid would tend to acidify the solution, buffering at pH 7 isn’t how the equivalence point behaves in a weak-acid/strong-base titration, and water’s own autoionization isn’t the driving factor in this basic shift.