Question

The $pOH$ at which $0.01 \,M \,Co ^{2+}$ ions in solution precipitate down as $Co ( OH )_{2}$ is _______.
$\left( K _{ sp }\right.$ of $Co ( OH )_{2}$ is $\left.2.5 \times 10^{-16}\right)$

Answer 1

$Co ( OH )_{2}( s ) \rightleftharpoons Co ^{2+}( aq )+2 OH ( aq )$

$\left[ Co ^{2+}\right][\bar{ O } H ]^{2}= K _{ sp }=2.5 \times 10^{-16}$

$(0.01)[\bar{ O } H ]^{2}=2.5 \times 10^{-16}$

$[\bar{ O } H ]^{2}=\frac{2.5 \times 10^{-16}}{0.01}=2.5 \times 10^{-14}$

$[\bar{ O } H ]=\sqrt{2.5 \times 10^{-14}}$

$=1.58 \times 10^{-7} M$
$pOH = -log \, 1.58 \times 10^{-7} = 6.8$