[Olympiad] The solubility product (Ksp) of Magnesium hydroxide Mg(OH)₂ is 1.8 × 10⁻¹¹. At what pH will a precipitate begin to form in a 0.01 M solution of MgCl₂?
- 8.45
- 9.15
- 9.63
- 10.21
📝 Detailed explanation
Mg(OH)₂ ⇌ Mg²⁺ + 2OH⁻.Ksp = [Mg²⁺][OH⁻]².
1.8 × 10⁻¹¹ = (0.01)[OH⁻]² => [OH⁻]² = 1.8 × 10⁻⁹.
[OH⁻] = 4.24 × 10⁻⁵ M.
pOH = -log(4.24 × 10⁻⁵) = 4.37.
pH = 14 - 4.37 = 9.63.