${\mathrm{p}}^{\mathrm{H}}\hspace{0.17em}\hspace{0.17em}\mathrm{of}\hspace{0.17em}\hspace{0.17em}0.005\mathrm{M}\mathrm{\hspace{0.17em}}\mathrm{weak}\hspace{0.17em}\hspace{0.17em}\mathrm{monobasic}\hspace{0.17em}\hspace{0.17em}\mathrm{acid}\hspace{0.17em}\hspace{0.17em}\mathrm{is}\hspace{0.17em}\hspace{0.17em}\_\_\_\mathrm{\hspace{0.17em}}\left({\mathrm{K}}_{\mathrm{a}}\mathrm{\hspace{0.17em}}=\mathrm{\hspace{0.17em}}2 \mathrm{x} {10}^{-4}\right)$

$\begin{array}{l}\left[{\mathrm{H}}^{+}\right]=\sqrt{{\mathrm{ck}}_{\mathrm{a}}}\\ =\sqrt{\left(5\times {10}^{-3}\right)\left(2\times {10}^{-4}\right)}\\ ={10}^{-3} \\ {\mathrm{p}}^{\mathrm{H}}=3\end{array}$