Question

Hydrolysis constants of two salts $KA$ and $KB$ of weak acids $HA$ and $HB$ are ${10}^{-8}$ and ${10}^{-6}$. The dissociation constant of a third acid $HC$ is${10}^{-2}$. The order of acidic strengths of the three acids will be:

• A. $HA>HB>HC$
• B. $HB>HA>HC$
• C. $HC>HA>HB$
• D. $HA=HB=HC$

Answer 1

The correct option is C $HC>HA>HB$

Given, $K_h$ of the salts of weak acid.
From relation between $K_h$ and $K_a$ we will calculate the $K_a$ value and then compare the strength of the acids.
$K_h=\frac{K_w}{K_a}$
$K_a$ value for HA = $\frac{K_w}{K_a}$
= $\frac{{10}^{-14}}{10-8}$ = ${10}^{-6}$
Similarly $K_a$ value for HB = $\frac{K_w}{K_a}$
=$\frac{{10}^{-14}}{10-6}$ = ${10}^{-8}$
and $K_a$ value for HC is given as ${10}^{-2}$
So larger the $K_a$ value, higher its strength
Hence correct order is
$HC>HA>HB$