Question

The ratio of dissociation constant of two weak acids $HA$ and $HB$ is $5$ The two acids will have same $pH$. When molar concentration ratio is______.

Answer 1

Answer: 0.2

$K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{[HA]}=\frac{{\left[H^{+}\right]}^2}{[HA]}$

$\left(\because \left[H^{+}\right]=[A^{-}]\right)$

and $K_b=\frac{\left[H^{+}\right]\left[B^{-}\right]}{[HB]}=\frac{{\left[H^{+}\right]}^2}{[HB]}\left(\because \left[H^{+}\right]=\left[B^{-}\right]\right)$

Also $H^{+}$ are same

$\Rightarrow \frac{K_a}{K_b}=\frac{[HB]}{[HA]}=\frac{5}{1}=0.2$