Question

The pH at which Mg(OH)\(_2\) \([K_{sp} = 1 \times 10^{-11}]\) begins to precipitate from a solution containing 0.10 M Mg\(^{2+}\) ions is \( \_\_\_\_ \).

Answer 1

Correct Answer: 9

Precipitation occurs when \( Q_p = K_{sp} \). The solubility product expression is:

\( [\text{Mg}^{2+}][\text{OH}^-]^2 = K_{sp} \)

Given:

\( 0.1 \times [\text{OH}^-]^2 = 10^{-11} \)

Solving for \( [\text{OH}^-] \):

\( [\text{OH}^-] = 10^{-5} \)

Then,

\( \text{pOH} = 5 \Rightarrow \text{pH} = 9 \)