Question

In hydrolysis of a salt of weak acid and strong base, $A^{-}+H_{2}O\rightleftharpoons HA+O{H}^{-}$,the hydrolysis constant $\left(K_h\right)$ is equal to

• A. $\frac{K_w}{K_a}$
• B. $\frac{K_w}{K_b}$
• C. $\sqrt{\frac{K_a}{C}}$
• D. $\frac{K_w}{K_a\times K_b}$

Answer 1

Answer: (A)

In hydrolysis of a salt of weak acid and strong base
$A^{-}+H_{2}O\rightleftharpoons HA+O{H}^{-}$
the hydrolysis constant $K_h$
$K_h=\frac{[HA]\left[O{H}^{-}\right]}{\left[A^{-}\right]}$
(According to law of mass action)
( $\because$ conc. of water remains constant)
For the ionisation of weak acid $HA$,
we has $HA\rightleftharpoons H^{+}+A^{-}$
$K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{[HA]}$
Also we know that
$\frac{K_w}{K_a}=\frac{\left[H^{+}\right]\left[O{H}^{-}\right][HA]}{\left[H^{+}\right]\left[A^{-}\right]}$
$\frac{K_w}{K_a}=\frac{\left[O{H}^{-}\right][HA]}{\left[A^{-}\right]}=K_h$