common ion effect

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Common Ion Effect
” The decrease in the  degree of dissociation of a weak electrolyte  on addition of a strong electrolyte containing a common ion . This is known as common ion effect”
Dissociation of  acetic acid [CH3COOH ]decreases in presence of sodium acetate

CH3COONa—–> CH3COO + Na+
Weak Electrolyte      Common ion
A/c to law of mass action,

CH3COONa is a strong electrolyte, it is almost completely ionised into   acetate ion [CH3COO ]&  sodium ion [Na+ ].When CH3COONa is added to the solution of acetic Acid the concentration of CH3COO increases.

Concentration of (H+) ions depend upon the concentration of (CH3COO)
when Concentration  of (CH3COO) increases then concentration of H+ decreases. It means dissociation of CH3COOH is decreased.
Ex.2- Dissociation of NH4OH decreases in presence of NH4Cl. Explain

NH4Cl               —–>  NH4+          +  Cl

NH4OH               —–>   NH4+       +   OH

weak electrolyte            Common ion
A/c to Law of mass action-

NH4Cl is a strong electrolyte & ionises to give   NH4+ &  Cl. After the addition of NH4OH, the Concentration of  NH4+ increases .
Concentration of (OH) depends upon  the concentration of NH4+. So concentration of [OH] decreases. Hence dissociation of NH4OH decreases.

Hence in presence of common ion, dissociation of weak electrolyte decreases.