The pH at which Mg ( OH ) _ 2 [ K _ sp = 1 × 10 ^ - 11 ] begins to...

Precipitation when ${Q}_{} = {K}_{sp}$

$Q =$Reaction quotient, it is the ratio of products to reactants at any time of reaction.

${K}_{sp} =$Solubility product.

$Mg{\left(OH\right)}_{2} ⇋ {Mg}^{2 +} + 2{OH}^{-}$

\left[{Mg}^{2 +}\right]{\left[{OH}^{-}\right]}^{2} = {10}^{- 11}$</p><p>$0 . 1 \times {\left[{OH}^{-}\right]}^{2} = {10}^{- 11} ⇒ \left[{OH}^{-}\right] = {10}^{- 5}$</p><p>$⇒ pOH = 5 ⇒ pH = 14 - 5 = 9