Three drops of a 1.0 mol dm⁻³ solution of HCl were added to 20 cm³ of a solution of pH 6.4. The pH of the resulting solution will be

Correct answer: Less than 6.4

Explanation: Adding a strong acid (HCl) increases the hydrogen-ion concentration, so the pH must decrease. Here, even a few drops of 1.0 mol dm⁻³ HCl contain far more H⁺ than was originally present in 20 cm³ of a pH 6.4 solution (which has only about 4×10⁻⁷ mol dm⁻³ H⁺). Therefore, the resulting solution is more acidic, with pH < 6.4 (often much lower, unless the solution is buffered).

• Close to that of pure water — Incorrect; adding strong acid won’t give pH ≈ 7.
• Less than 6.4 — Correct.
• Greater than 6.4 — Incorrect; that would mean becoming less acidic.
• Unaltered — Incorrect; pH changes on adding strong acid (no buffer stated).