Which of the following will produce a solution with pH less than 7 at equivalence point?

Correct answer: Ionic — HCl + Mg(OH)₂ produces a solution with pH < 7 at equivalence.

Explanation: At the equivalence point, the nature of the salt formed determines the pH:

• HCl + Mg(OH)₂ → forms MgCl₂ (salt of a strong acid and a weak base). The Mg²⁺ ion hydrolyses slightly in water, producing H⁺ and giving an acidic solution (pH < 7).
• HNO₃ + NaOH → NaNO₃ (strong acid + strong base) gives a neutral solution at equivalence (pH ≈ 7).
• H₂SO₄ + KOH → At full (second) equivalence, K₂SO₄ is formed (strong acid + strong base overall), so the solution is approximately neutral. (Note: the first equivalence would give KHSO₄, which is acidic, but “equivalence point” here refers to complete neutralization.)
• HNO₃ + KOH → KNO₃ (strong acid + strong base) gives a neutral solution at equivalence.