Applications of Solubility product-Part 2

Applications of Solubility product

source : genchem.rutgers.edu

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

(soap)

NaCl  ————> Na⁺  + Cl^–

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

Ionic product of [ C₁₅H₃₁COO ^–] [Na⁺]   > K_sp 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^–] > K_sp of NaCl

Therefore pure NaCl precipitates out from the solution.

iii) Precipitation of  III^rd group metal hydroxides –

III^rd 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  > K_sp of  III^rd group metal hydroxides

Thus only III^rd group metal hydroxides get precipitated in III^rd group.