Salt hydrolysis

source : SlideServe

## Hydrolysis of salt of weak acid and weak base-

In such salts , both the cation of weak base and anion of weak acid will undergo hydrolysis simultaneously .

#### NH4+ + CH3COO– + H2O ———> NH4OH + CH3COOH

According to law of mass action,

Hydrolysis  constant ‘Kh‘ = [NH4OH][CH3COOH] / [NH4+][CH3COO] [H2O]

Water is present in excess hence can be regarded as constant.

#### Kh = [NH4OH][CH3COOH] / [NH4+][CH3COO–]          ————eq 1

CH3COOH   ⇌  CH3COO  +  H+

#### Ka = [CH3COO–][H+] / [CH3COOH]         ———– eq 2

NH4OH     ⇌   NH4+  + OH

#### Kb = [NH4+][OH–] / [NH4OH ]           ———— eq 3

H2O    ⇌  H+  + OH

#### Kw = [H+][OH–]        ———– eq 4

By eq (2) ,(3) and (4)

Kw / Ka.Kb = [NH4OH][CH3COOH] [H+][OH]  / [NH4+][OH]  [CH3COO]  [H+]

#### Kw / Ka.Kb =  [NH4OH][CH3COOH] / [NH4+][CH3COO–]         ——– eq 5

comparing eq (1) and (5)

## Degree of hydrolysis and hydrolysis constant-

NH4 +    +  CH3COO   +     H2O     ⇌     NH4OH  +  CH3COOH

weak ion    weak ion

Before hydrolysis      C                  C                                                    0                   0

After hydrolysis      C ( 1-h)          C ( 1-h)                                        Ch                 Ch

#### Kh = [NH4OH][CH3COOH] / [NH4+][CH3COO–]

Water is present in excess hence can be regarded as constant.

### Kh = h2/(1-h)2

because (1-h) is nearly equal to one therefore ,

Kh =  h2

h = Kh

h = degree of hydrolysis

because ,

## Expression for the pH of the salt solution –

CH3COOH   ⇌  CH3COO  +  H+

#### [H+]  = Ka [CH3COOH]  / [CH3COO–]

[H+] = Ka. Ch/ C(1-h)

[H+] = Ka. h / (1-h)

because (1-h) is nearly equal to one therefore ,

[H+] = Ka. h

because ,

### [H+]  = √ (Kw . Ka / Kb)

Taking log and reverting the sign through out,

-log [H+] = – 1/2 log Kw  -1/2 log Ka  + 1/2 log Kb

-log [H+] = pH , – log Kw =  pKw , – log Ka = pKa , – log Kb = pKb

pH =  1/2 pKw  + 1/2 log pKa  –  1/2 log pKb

### ” The pH of salt of weak acid and weak base is independent of concentration of salt.

When  pKa   > pKb ,  than  solution will be alkaline and pH value will be more than 7. In case  pKb   > pKa ,  than e solution will be acidic and pH value will be less than 7.