Salts of weak acids and weak bases

When salt BA of weak acid HA and weak base BOH is dissolved in water, it dissociates completely as

The hydrolysis reaction involves the interaction of both the ions of the salt with water,

The solution may turn out acidic, basic or neutral depending on the relative strength of weak base and weak acid formed in the hydrolysis.

i. if, $K_a > K_b$, the solution will be acidic.

ii. if, $K_a < K_b$, the solution will be basic.

iii. if, $K_a = K_b$, the solution will be neutral.

i. Salt of weak acid and weak base for which $K_a > K_b$. $\mathrm{NH_4F}$ is a salt of weak acid HF ($K_a = 7.2 \times 10^{-4}$) and weak base $\mathrm{NH_4OH}$ ($K_b = 1.8 \times 10^{-5}$). Here, $K_a$ is greater than $K_b$. The salt hydrolyses as

The acid HF is slightly stronger than base $\mathrm{NH_4OH}$. The two ions react with water as

The $\mathrm{NH_4^{+}}$ ions hydrolyse to a slightly greater extent than the $\mathrm{F^{-}}$ ions. That means the reaction (3.19) produces more $\mathrm{H_3O^{+}}$ ions than the $\mathrm{OH^{-}}$ ions produced in reaction (3.20). In other words, $\mathrm{NH_4^{+}}$ ions are slightly stronger as acid than $\mathrm{F^{-}}$ ions as base. The solution of $\mathrm{NH_4F}$ is thus only slightly acidic and turns blue litmus red.

ii. Salt of weak acid and weak base for which $K_a < K_b$ : $\mathrm{NH_4CN}$ is the salt of weak acid HCN ($K_a = 4.0 \times 10^{-10}$) and weak base $\mathrm{NH_4OH}$ ($K_b = 1.8 \times 10^{-5}$) showing that $K_a < K_b$. When $\mathrm{NH_4CN}$ is dissolved in water, it hydrolyses as

The base $\mathrm{NH_4OH}$ is stronger than the acid HCN. The ions of the salt react with water as,

The $\mathrm{CN^{-}}$ ions hydrolyse to a greater extent than $\mathrm{NH_4^{+}}$ ions. The reaction (3.22) produces more $\mathrm{OH^{-}}$ ions than the $\mathrm{H_3O^{+}}$ ions produced in reaction (3.21). The solution of $\mathrm{NH_4CN}$ is, basic and turns red litmus blue.

iii. Salt of weak acid and weak base for which $K_a = K_b$. $\mathrm{CH_3COONH_4}$ is a salt of weak acid, $\mathrm{CH_3COOH}$ ($K_a = 1.8 \times 10^{-5}$) and weak base, $\mathrm{NH_4OH}$ ($K_b = 1.8 \times 10^{-5}$).

When the salt $\mathrm{CH_3COONH_4}$ is dissolved in water, it undergoes hydrolysis : …