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

$K _{ a }=\frac{\left[ H ^{+}\right]\left[ A ^{-}\right]}{[ H A]}=\frac{\left[ H ^{+}\right]^{2}}{[ H A]}$

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

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

Also $H ^{+}$ are same

$\Rightarrow \frac{ K _{ a }}{ K _{ b }}=\frac{[ HB ]}{[ HA ]}=\frac{5}{1}=0.2$