Titrating a weak base with a strong acid, such as NH3 with HCl, yields an equivalence point with pH less than 7 because the conjugate acid NH4+ is present.

• A. pH > 7
• B. pH ~ 7
• C. pH < 7
• D. pH cannot be predicted

Answer: (C)

When a weak base is titrated with a strong acid, the species present at the equivalence point is the conjugate acid of the base. For ammonia, that conjugate acid is ammonium ion (NH4+). NH4+ is a weak acid and will donate protons to water: NH4+ + H2O ⇌ NH3 + H3O+. This hydrolysis produces H3O+ in the solution, making it acidic. Because of this, the pH at the equivalence point is less than 7. The exact value depends on the concentration, but it is predictably acidic rather than neutral or basic.