The ionization constant of water at ${25}^{\circ }\mathrm{C}$ is

$$\begin{gathered} K_{\mathrm{i}}=\frac{\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]\left[{\mathrm{OH}}^{-}\right]}{\left[{\mathrm{H}}_2\mathrm{O}\right]} \\ {K}_{\mathrm{i}}=\frac{{\left({10}^{-7}\mathrm{M}\right)}^2}{55.56\mathrm{M}} \\ {K}_{\mathrm{i}}=1.80\times {10}^{-16}\mathrm{M} \end{gathered}$$

A quantity known as the dissociation constant—symbol K—depends on the equilibrium between ions and non-ionized molecules in a solution or liquid.