The pH of a solution of salt formed from a weak acid and a weak base decreases with

For a salt formed from a weak acid and a weak base, pH of the solution is given by

$\mathrm{pH}=\frac{1}{2}\left(\mathrm{p}{K}_{\mathrm{w}}^{\circ }+\mathrm{p}{K}_{\mathrm{a}}^{\circ }-\mathrm{p}{K}_{\mathrm{b}}^{\circ }\right)$