Calculate the pOH of a solution at 25°C that contains 1x 10^-10 M of hydronium ions, i.e., H₃O⁺ 

$\text{ Given }{\mathrm{H}}_3{\mathrm{O}}^{+}\text{ion concentration }=1\times {10}^{-10}$
$\begin{array}{l}\mathrm{pH}=-\log \left[{\mathrm{H}}^{+}\right],\mathrm{pH}=-\log \left[1\times {10}^{-10}\right]\\ \mathrm{pH}=+10\log 10,\mathrm{pH}=10\end{array}$
$\mathrm{We} \mathrm{know} \mathrm{that}, \mathrm{pH}+\mathrm{pOH}=14 ----\left(\mathrm{I}\right)$
Put the value of pH in eq. (i)
$\begin{array}{l}10+{\mathrm{P}}^{\mathrm{OH}}=14\\ {\mathrm{P}}^{\mathrm{OH}}=4\end{array}$