The dissociation constant of weak acid HA is  $4.9\times {10}^{-8}$. After making the necessary approximations, calculate pH of 0.1 M acid.

For weak acid $K_a =\mathrm{C} {\alpha }^2.$

$\therefore \mathrm{\alpha } = \sqrt{\frac{{\mathrm{K}}_{\mathrm{a}}}{\mathrm{C}}} = \sqrt{\frac{4.9\times {10}^{-8}}{0.1}} = 7\times {10}^{-4}$

$$\begin{gathered} {\text{[ H}}^{+}\text{]=Cα} \\ {\text{P}}^{\mathrm{H}}=-\log [ {\mathrm{H}}^{+}]=-\log \mathrm{C\alpha } \end{gathered}$$

     $=-\log 7\times {10}^{-4}\times {10}^{-1^{ }}$

      = 4.1549 = 4.155