Applications of Solubility product

Applications of Solubility product

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Applications of Solubility product-

i) Salting out of soap –

The important applications of Solubility product is Salting out of soap. Soap is sodium salt of higher fatty acid . It is precipitated from the solution by adding concentrated solution of NaCl( sodium chloride) .

C15H31COONa ————–>  C15H31COO      +   Na+

sodium palmitate


NaCl  ————> Na+  + Cl

NaCl  is strong electrolyte , it ionises completely in solution then concentration of sodium ions  (Na+) increases.

Ionic product of [ C15H31COO ] [Na+]   > Ksp of  C15H31COONa (soap)

Hence soap precipitates out from the solution.

ii) Purification of common salt-

Natural common salt contains soluble and insoluble impurities . When saturated solution of common salt is prepared then insoluble impurities are filtered off.

HCl gas is passed in saturated solution of NaCl.

NaCl ———> Na +   + Cl

HCl ———> H +   + Cl

Both gives chloride ions( Cl)in solution therefore concentration of Cl increases. Hence,

ionic product of  [Na +]  [ Cl] > Ksp of NaCl

Therefore pure NaCl precipitates out from the solution.

iii) Precipitation of  IIIrd group metal hydroxides –

IIIrd group metal hydroxides  [ Al(OH)3 , Fe(OH)3 , Cr(OH)3 ] get precipitated with ammonium hydroxide (NH4OH) in presence of ammonium chloride (NH4Cl).

NH4Cl ——–> NH4 +  + Cl

NH4OH   ⇌  NH4+  + OH

Because in presence of NH4Cl ( it is strong electrolyte and gives NH4 + ), dissociation of weak electrolyte NH4OH decreases due to common ion effect. Thus the concentration of  [OH]  must be lowered. Thus

Ionic product of [ M+++] [OH] 3  > Ksp of  IIIrd group metal hydroxides

Thus only IIIrd group metal hydroxides get precipitated in IIIrd group.