${\mathrm{p}}^{\mathrm{H}}$ of 0.1M weak monobasic acid is 4. Ionisation constant of weak acid is

For a weak monobasic acid,  

$\left[{\mathrm{H}}^{+}\right] =\sqrt{{\mathrm{CK}}_{\mathrm{a}}}$

${10}^{-4}=\sqrt{0.1\times {\mathrm{K}}_{\mathrm{a}}}$

${10}^{-8}=0.1{\mathrm{K}}_{\mathrm{a}}$

${\mathrm{K}}_{\mathrm{a}}=\frac{{10}^{-8}}{0.1}$

${\text{K}}_a={10}^{-7}$