Question

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

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

Answer 1

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

The acid dissociation constant $K_a$ is the ratio of the ionic product of water $K_w$ to the hydrolysis constant $K_h$.
For acid HA, $K_a=\frac{K_w}{K_h}=\frac{{10}^{-14}}{{10}^{-8}}={10}^{-6}$.
For acid HB, $K_a=\frac{K_w}{K_h}=\frac{{10}^{-14}}{{10}^{-6}}={10}^{-8}$.
For acid HC, $K_a={10}^{-2}$.

As the value of the acid dissociation constant $K_a$ increases, the acid strength increases.

Hence, the decreasing order of the acid strength is $HC>HA>HB$.