Question

The hydroxide ion has the formula OH−. The solubility-product constants for three generic hydroxides are given here.

Generic hydroxide Ksp

XOH 1.30×10−8

Y(OH)2 4.00×10−10

Z(OH)3   9.10×10−15

Use these values to answer the following questions. The removal of an ion is sometimes considered to be complete when its concentration drops to 1.00×10−6 M. What concentration of hydroxide would cause Y2+ to "completely" precipitate from a solution?

Express your answer with the appropriate units.

[ O H − ] =

Answer 1

[OH^-] = 0.0200 M

Explanation

K_sp Y(OH)₂ = 4.00 x 10^-10

Also K_sp Y(OH)₂ = [Y²⁺][OH^-]²

where [Y²⁺] = concentration of Y²⁺ after complete precipitation = 1.00 x 10^-6 M

[OH^-] = concentration of OH^-

Substituting the values

4.00 x 10^-10 = (1.00 x 10^-6 M) * [OH^-]²

[OH^-]² = (4.00 x 10^-10) / (1.00 x 10^-6 M)

[OH^-]² = 4.00 x 10^-4 M²

[OH^-] = (4.00 x 10^-4 M²)^1/2

[OH^-] = 0.0200 M