The pH of a 0.01 M weak acid $\mathrm{HX}\left(K_{a}=4 imes 10^{-10} ight)$ is found to be 5 . No

Question

The pH of a 0.01 M weak acid $\mathrm{HX}\left(K_{a}=4 	imes 10^{-10}ight)$ is found to be 5 . Now the acid solution is diluted with excess of water so that the pH of the solution changes to 6 . The new concentration of the diluted weak acid is given as $x 	imes 10^{-4} \mathrm{M}$. The value of $x$ is $\_\_\_\_$ . (nearest integer)\

Professor PC Thomas Classes · Answer

(25) : Given: $\left(K_{a}=4 	imes 10^{-10}ight)$ After dilution $$ \begin{aligned} & \mathrm{pH}=6 \ & {\left[\mathrm{H}^{+}ight]=10^{-6}=\sqrt{K_{a} C}} \ & 10^{-6}=\sqrt{4 	imes 10^{-10} 	imes C} \ & C=\frac{10^{-12}}{4 	imes 10^{-10}} \ & C=0.25 	imes 10^{-2}=25 	imes 10^{-4} \mathrm{M} \ & 	herefore \quad x=25 \end{aligned} $$