Question

The ksp of al(oh)3 is 2 x 10–32. at what ph will a 0.4 m al3+ solution begin to show precipitation of al(oh)3?

Answer 1

Answer:

At pH = 6.19, solution begin to show precipitation

Explanation:

Ksp of Al(OH)₃ is:

Al(OH)₃(s) ⇄ Al³⁺(aq) + 3 OH⁻(aq)

At equilibrium:

Ksp = 2x10⁻³² = [Al³⁺] [OH⁻]³

0.4M Al(OH)₃ produce:

[Al³⁺] = X

[OH⁻] = 3X

Replacing:

2x10⁻³² = [X] [3X]³

2x10⁻³² = 27X⁴

7.41x10⁻³³ = X⁴

5.217x10⁻⁹ = X

Thus:

[OH⁻] = 3X = 3×5.217x10⁻⁹

[OH⁻] = 1.565x10⁻⁸M

pOH = - log [OH⁻] = 7.805

pH = 14 - pOH

pH = 6.19

Thus, at pH = 6.19, solution begin to show precipitation.