At $27^{\circ} \mathrm{C}$, the degree of dissociation of $\mathrm{H} A$ (weak acid) in 0.5 M of its solution is $1 \%$. The concentrations of $\mathrm{H}_3 \mathrm{O}^{+}, A^{-}$and $\mathrm{H} A$ at equilibrium (in $\mathrm{mol} \mathrm{L}^{-1}$ ) are respectively.

100 mL of $0.1 \mathrm{M} \mathrm{H} A$ (weak acid) and 100 mL of 0.2 M $\mathrm{Na} A$ are mixed. What is the pH of resultant solution? ( $K_{\mathrm{a}}$ of $\mathrm{H} A$ is $10^{-5}: \log 2=0.3$ )