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  4. What is the pH at which Mg(OH)2 begins to precipitate from a solution containing 0.1 M Mg2+ ions? [Ksp for Mg(OH)2 = 1.0 × 10-11]

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Q. What is the $pH$ at which $Mg(OH)_2$ begins to precipitate from a solution containing $0.1\,M \,Mg^{2+}$ ions? $[K_{sp}$ for $Mg(OH)_2 = 1.0 \times 10^{-11}]$

Equilibrium

Solution:

$K_{sp}$ for $Mg(OH)_2 = 1.0 \times 10^{-11}$
$\left[Mg^{2+}\right]\left[OH^{-}\right]^{2}=\left(0.1\right)\times\left[OH^{-}\right]^{2}$
$=1.0\times10^{-11}$
$\left[OH^{-}\right]^{2}=\frac{1\times10^{-11}}{0.1}=10^{-10}$
$\left[OH^{-}\right]=10^{-5}$ ;
$\left[H^{+}\right]=\frac{10^{-14}}{10^{-5}}=10^{-9}$
$pH=9$