Salt of a Strong Acid and Weak Base

Here, cation B⁺ undergoes hydrolysis to give free H⁺ ions. Therefore the resulting solution will be acidic in character having pH less than 7. For example,

Other examples of this type of salts are NH₄Cl, CuSO₄, AlCl₃, etc.

The aqueous solution of a salt of strong acid and weak base is acidic. For the general reaction:

or

This is called cationic hydrolysis.

Hydrolysis constant

The hydrolysis constant may be written as:

(iv)

For weak base BOH,

Multiplying equation (iv) and (v) and dividing by equation (vi), we get

The hydrolysis constant 'K_h', of the salt is inversely proportional to the dissociation constant, 'K_b' of the weak base. Therefore, the weaker the base, the greater is the hydrolysis constant of the salt.

Degree of hydrolysis

If the original concentration of the salt in the solution is 'c' mol/litre and 'h' is the degree of hydrolysis at that concentration, then

Initial concentration	c	0	0
Concentration at eqilibrium	c(1-h)	ch	ch

 

K_h = ch²

pH of the hydrolysed salt solution [H⁺] = ch

 

 

Now, pH = -log [H⁺]

Now, -log K_w = pK_w - log K_b = pK_b

Knowing the molar concentration 'c' of the solution, K_b and K_w the pH of the solution can be calculated.