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Question

For the solution of salt made from weak acid and strong base, give the relationship between K_b, K_w and K_a, the amount of hydrolysis and the formula to determine the pH of solution.

Vidyadip · Accepted Answer

An example of a salt made from a weak acid and strong base is $CH _3 COONa$. In its solution, acetate ion hydrolysed with water to form acetic acid and $OH ^{-}$. Hence it is called anionic hydrolysis. $
CH_3 COO^{-}(aq)+H_2 O(l) \rightleftharpoons CH_3 COOH(aq)+OH^{-}(aq)
$
Due to $OH ^{-}$ions, the solution is alkaline and pH of solution is more than 7 .
For this hydrolysis :
$
K_{w}=\text { ionic product of water }
$
(i)
$
\begin{aligned}
K_{h}=\frac{K_{w}}{K_{a}} \quad K_{h} & =\text { hydrolysis constant } \\
K_{a} & =\text { acid dissociation }
\end{aligned}
$
(ii) Degree of hydrolysis :
$
h=\sqrt{\frac{K_{h}}{c}} \text { or } h=\sqrt{\frac{K_{w}}{K_{a \times c}}}
$
(iii) pH of solution :
$
pH=\frac{1}{2}\left(pK_{w}+pK_{a}+\log c\right)
$

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