pH Values of Acid/Base Solutions

Steps:

• HCl is a strong acid; [H⁺] = 0.1 mol/dm³, so pH = -log(0.1) = 1.
• NaOH is a strong base; [OH⁻] = 0.1 mol/dm³, pOH = 1, pH = 14 - 1 = 13.
• NaCl is a neutral salt from strong acid and base, so pH = 7.
• NH₃ is a weak base; for 0.1 mol/dm³, approximate pH ≈ 11 using Kb = 1.8 × 10⁻⁵.

Why C is correct:

• C matches pH = 11 (weak base NH₃), 1 (strong acid HCl), 7 (neutral NaCl), 13 (strong base NaOH) per acid-base strength definitions.

Why the others are wrong:

• A: NH₃ pH 7 (should be basic), NaCl pH 13 (should be neutral), NaOH pH 11 (should be 13).
• B: NaOH pH 11 (confuses strong base with weak base value).
• D: Swaps acidic/basic roles; HCl pH 11 (should be 1), NaOH pH 1 (should be 13), NH₃ pH 13 (overestimates weak base).

Final answer: C