Salt of Weak Acid and a Weak Base

In this case both the cation and anion undergo hydrolysis to the same or different extents. The resulting solution may be neutral, acidic or

basic depending upon the relative strengths of acids and bases. The hydrolysis may be written as:

Some common examples are CH₃COONH₄, (NH₄)₂CO₃, AlPO₄, etc.

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

or

This involves anionic and cationic hydrolysis.

Hydrolysis constant

The hydrolysis constant may be written as:

(vi)

for weak acid,

For weak base BOH,

Also, K_w = [H⁺] [OH^-]

Multiplying equation (vi) by equation [H⁺] [OH^-], we get

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

 

K_h = h²

or

Here the degree of hydrolysis is independent of the concentration of the solution. The weaker the acid and the base, the greater is the degree of hydrolysis of the salt.

pH of the hydrolysed salt solution

Now, pH = -log [H⁺]

 

 

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

It is clear from the above equation that pH of the solution will depend upon the pK value of the acid and the base.

• If pK_a < pK_b, pH of the solution will be less than 1/2 pK_W and consequently the solution will be acidic.

• If pK_a > pK_b, then pH of the solution will be more than 1/2 pK_W and hence the solution will be alkaline.

• If pK_a = pK_a, pH of the solution will be equal to   pKw and hence the solution will be neutral.