Question

The degree of hydrolysis for a salt of strong acid and weak base
This question has multiple correct options
A) Is independent of dilution
B) Increases with dilution
C) Increases with decrease in $$K_b$$ of the bases
D) Decreases with decrease in temperature

Answer 1

Hint:The degree of hydrolysis gives the fraction of the salt molecules that undergo hydrolysis when dissolved in water. Percent hydrolysis gives the number of the salt molecules that undergo hydrolysis when 100 molecules of salt are dissolved in water.

Complete answer:
We can quantitatively express the hydrolysis of salts in terms of the degree of hydrolysis (h).
We can calculate the degree of hydrolysis by dividing the number of hydrolyzed salt molecules to the total number of salt molecules.
The total number of salt molecules include the number of hydrolyzed salt molecules and the number of unhydrolyzed salt molecules.
$$\text{The degree of hydrolysis = }{\displaystyle \frac{\text{the number of hydrolyzed salt molecules}}{\text{the total number of salt molecules}}}$$
In the above expression, We can replace the number of molecules with concentration
$$\text{The degree of hydrolysis = }{\displaystyle \frac{\text{the concentration hydrolyzed salt molecules}}{\text{the total concentration of salt molecules}}}$$
Usually We have a small value of the degree of hydrolysis.
For a salt of strong acid with weak base, with increase in either temperature or dilution or both, the degree of hydrolysis increases.
For a salt of weak acid with weak base, there is no correlation between the degree of hydrolysis and dilution.
We can use the following expression to calculate the degree of hydrolysis for salt of strong acid and weak base
$$\text{h = }\sqrt{{\displaystyle \frac{K_w}{K_b\times C}}}$$
Here, h represents the degree of hydrolysis of the salt, $$K_w$$ represents the ionic product of water, $$K_b$$ represents the base dissociation constant and C represents the salt concentration.
With decrease in the value of the base dissociation constant $$K_b$$ , the value of the degree of hydrolysis increases.
Hence, the degree of hydrolysis for a salt of strong acid and weak base increases with dilution, increases with decrease in $$K_b$$ of the bases and decreases with decrease in temperature.

Hence, the options (B), (C) and (D) are the correct answers.

Note: Ammonium chloride is a salt of weak base ammonia and strong acid hydrochloric acid. When such salts are dissolved in water, the ammonium cation undergoes hydrolysis to form hydronium ions. Hence, such salt solutions are acidic with pH less than 7.
$${\text{NH}}_4^{+}\text{ + }{\text{H}}_2\text{O }\to \text{ N}{\text{H}}_3\text{ + }{\text{H}}_3{\text{O}}^{+}$$