In a saturated solution of Al(OH)₃, the concentration of OH⁻ is 2.6 x 10⁻⁵ mol/L. The concentration of Al³⁺ in mol/ L is which of the following?

Question

1. 7.8 x 10⁻⁵
2. 8.7 x 10⁻⁶
3. 7.8 x 10⁻¹⁵
4. 2.6 x 10⁻⁵
5. 5.2 x 10⁻⁵

Answer 1

Al(OH)₃(s) <==> Al³⁺(aq) + 3OH^-(aq) ... dissolution of solid Al(OH)₃

Note that the [OH^-] in solution will be 3 times that the [Al³⁺] in solution. Or put another way, the [Al³⁺] will be 1/3 the [OH^-].

Thus...

[Al³⁺] = 1/3 x 2.6x10^-5 M

[Al³⁺] = 8.7x10^-6 M (Answer #2)