Practice question
Question
A weak acid HY has Ka = 1.6 × 10⁻⁷ . What is the pH of a 0.02 M solution of HY ?
Explanation
For HY <=> H+ + Y- , Ka = (x²/0.02 - x) ≈ (x²/0.02) = 1.6 × 10⁻⁷ . Solving, x² = 3.2 × 10⁻⁹ , x = 1.79 × 10⁻⁴ , pH = -log(1.79 × 10⁻⁴) ≈ 3.75 .