Final answer:
The pH at the equivalence point in the titration of a weak acid with a strong base is greater than 7 due to the formation of a basic salt from the weak acid and the strong base.
Step-by-step explanation:
When a weak acid is titrated by a strong base, the pH at the equivalence point is always >7. This is because the weak acid partially dissociates in water to form conjugate base ions, which combine with the strong base to form water and a salt. The salt formed is of a weak acid, which consequently results in a basic solution at the equivalence point due to the hydrolysis of the conjugate base ions.
For example, if acetic acid (a weak acid) is titrated with sodium hydroxide (a strong base), the acetate ions from the acetic acid react with the sodium ions from the sodium hydroxide to form sodium acetate, which is a basic salt. This results in a pH greater than 7 at the equivalence point. Therefore, indicators like phenolphthalein or thymol blue, with pKin > 7.0, are appropriate for such titrations.