Question Detail
Question
A solution of a weak monoprotic acid of concentration 0.020 M has a pH of 3.20. Calculate the dissociation constant, Kₐ, of the acid.
Correct Answer
Option B is the correct answer.
Detailed Explanation
[H⁺] = 10⁻³·²⁰ = 6.31 × 10⁻⁴ M. For HA ⇌ H⁺ + A⁻, Kₐ = [H⁺]² / (C - [H⁺]) = (6.31×10⁻⁴)² / (0.020 - 0.000631) = (3.98×10⁻⁷) / (0.01937) = 2.05×10⁻⁵ M.
Hint
pH gives [H⁺]. Use the weak acid dissociation approximation or full formula.