pH of 0.01 M ${\mathrm{HS}}^{-}$ will be

(1) It is salt of weak acid and strong base.

${\mathrm{HS}}^{-} + {\mathrm{H}}_2\mathrm{O} \rightleftharpoons {\mathrm{H}}_2\mathrm{S} + {\mathrm{OH}}^{-}$

$\therefore \left[{\mathrm{OH}}^{-}\right] = \mathrm{Ch} = \sqrt{\frac{{\mathrm{K}}_{\mathrm{w}}\mathrm{C}}{{\mathrm{K}}_{\mathrm{a}}}}$

$\therefore \left[{\mathrm{H}}^{+}\right] = \frac{{\mathrm{K}}_{\mathrm{w}}}{\sqrt{\frac{{\mathrm{K}}_{\mathrm{w}}.\mathrm{C}}{{\mathrm{K}}_{\mathrm{a}}}}} = \sqrt{\frac{{\mathrm{K}}_{\mathrm{w}}{\mathrm{K}}_{\mathrm{a}}}{\mathrm{C}}}$

or    $\mathrm{pH}=1/2\left[{\mathrm{pK}}_{\mathrm{w}} +{\mathrm{pK}}_{\mathrm{a}} + \log \mathrm{C}\right]$