Question

K_sp of Al(OH)₃ is 1 × 10⁻¹⁵ M. At what pH does 1.0 × 10⁻³ M Al³⁺ precipitate on the addition of buffer of NH₄Cl and NH₄OH solution?

Answer 1

\[\ce{Al(OH)3 ⇌ Al^{3+}_{( aq)} + 3OH^-_{( aq)}}\]

K_sp = [Al³⁺] [OH^–]³

Al(OH)₃ precipitates when

[Al³⁺] [OH^–]³ > K_sp

(1 × 10⁻³) [OH^–]³ > 1 × 10⁻¹⁵

[OH^–]³ > 1 × 10⁻¹²

[OH^–] > 1 × 10⁻⁴ M

[OH^–] = 1 × 10⁻⁴ M

pOH = –log₁₀ [OH^–] = –log (1 × 10⁻⁴) = 4

pH = 14 – 4 = 10

Thus, Al(OH)₃ precipitates at a pH of 10