Calculate the $\text{pOH}$ of a solution at $25{}^{\circ }\text{C}$ that contains $1\times 10^{-10}\text{ M}$ of hydronium ions.

Given the concentration of hydronium ions $[H_3{O}^{+}]=1\times 10^{-10}\text{ M}$.First, calculate the $\text{pH}$:
$$\text{pH}=-\log [H_3{O}^{+}]=-\log (1\times 10^{-10})=10$$
At $25{}^{\circ }\text{C}$, the relationship between $\text{pH}$ and $\text{pOH}$ is:
$$\text{pH}+\text{pOH}=14$$
Substituting the value of $\text{pH}$:
$$10+\text{pOH}=14$$
$$\text{pOH}=14-10=4$$