In which titration would the pH at the equivalence point be less than 7?

• A. Strong acid with weak base
• B. Weak acid with strong base
• C. Strong acid with strong base
• D. Weak base with weak acid

Answer: (A)

When you reach the equivalence point, what’s left in solution is the conjugate partner of the titrated species, and its ability to either donate or accept protons sets the pH. If you’re titrating a strong acid into a weak base, the resulting salt contains the conjugate acid of the weak base. That ion is a weak acid and will donate H+, making the solution acidic. So the pH at equivalence is below 7 in this case.

For example, adding a strong acid to ammonia forms ammonium chloride; ammonium ions (the conjugate acid of ammonia) hydrolyze to release H+, pulling the pH below 7.

In contrast, titrating a weak acid with a strong base leaves the conjugate base of the weak acid, which is basic and drives the pH above 7 at equivalence. If both reactants are strong, the solution is neutral at equivalence. If both are weak, the pH at equivalence depends on the relative strengths and isn’t reliably below 7.

Therefore, the scenario where the equivalence point pH is less than 7 is when a strong acid is titrated into a weak base.