Question

The ratio of dissociation constant of two weak acids $HA$ and $HB$ is $4.$ At what molar concentration ratio, the two acids will have same $pH$ in separate solutions:

• A. 2
• B. 0.5
• C. 4
• D. 0.25

Answer 1

Answer: (D)

$K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{[HA]}=\frac{{\left[H^{+}\right]}^2}{[HA]}\left(\therefore \left[H^{+}\right]=\left[A^{-}\right]\right)$
and $K_b=\frac{\left[H^{+}\right]\left[B^{-}\right]}{[HB]}=\frac{{\left[H^{+}\right]}^2}{[HB]}\left(\therefore \left[H^{+}\right]=\left[B^{-}\right]\right)$
Also $H^{+}$are same
$\therefore \frac{K_a}{K_b}=\frac{[HB]}{[HA]}=\frac{4}{1}$
$\frac{[HA]}{[HB]}=\frac{1}{4}$