In a titration of a weak base with a strong acid, the pH at equivalence is:

• A. Less than 7
• B. Equal to 7
• C. Greater than 7
• D. Cannot be determined

Answer: (A)

When a weak base is titrated with a strong acid, the equivalence point leaves only the conjugate acid of the base in solution. That conjugate acid, BH+, is a weak acid in water and will donate protons: BH+ + H2O ⇌ B + H3O+. The tendency to donate H+ is quantified by Ka = Kw / Kb, so BH+ acidifies the solution. Because there is H3O+ present, the pH is less than 7. The stronger the original base (larger Kb), the stronger the conjugate acid and the more acidic the solution at equivalence, pushing pH further below 7.