The equilibrium constant $K_c$ for the reaction ${\mathrm{H}}_2\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{OH}}^{-}$ at ${25}^{\circ }\mathrm{C}$

$$\begin{gathered} K_c=\frac{\left[{\mathrm{H}}_3{\mathrm{O}}^{+}\right]\left[{\mathrm{OH}}^{-}\right]}{{\left[{\mathrm{H}}_2\mathrm{O}\right]}^2} \\ {K}_c={\left[\frac{\left({10}^{-7}\mathrm{M}\right)}{\left(55.56\mathrm{M}\right)}\right]}^2 \\ {K}_c=3.24\times {10}^{-18} \end{gathered}$$

The equation that represents the link between the rate of a chemical reaction and the concentrations of the reacting chemicals contains a proportionality constant called the rate constant, also known as the specific rate constant.