Chemistry questions

Question

At what pH will Fe(OH)₃(s) begin to precipitate from 5.15 × 10⁻³ M FeCl₃? Ksp for Fe(OH)₃ is 2.79 × 10⁻³⁹.

Answer 1

Fe(OH)₃(s) <==> Fe³⁺(aq) + 3OH^-(aq) .. equation for Fe(OH)₃ dissolving

Ksp = [Fe₃][OH^-]³

2.79x10^-39 = [5.15x10^-3][OH^-]³

[OH^-]³ = 5.42x10^-37

[OH^-] = 8.15x10^-13 M

pOH = -log [OH^-] = -log 8.15x10^-13 M

pOH = 12.09

pH = 14 - pOH = 14 - 12.09

pH = 1.91