By Sunil Bhardwaj

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Applications of Solubility Product:

(i) Determination of solubility

As we know the relation between solubility and solubility product is $$ S = { \left( \frac { { K }_{ sp } }{ { x }^{ x }{ y }^{ y } } \right) }^{ \frac { 1 }{ x+y } }\qquad \qquad \qquad ...(1)$$ if we know \({ K }_{ sp }\) (solubility product) we can easily calculate S.

(ii) Fractional precipitation

If in an aqueous solution of KI and KCl we add \(AgN{ O }_{ 3 }\) precipitate of AgI and AgCl will take place. $$ KI + AgN{ O }_{ 3 } \rightleftharpoons AgI \downarrow + KN{ O }_{ 3 } $$ $$ { K }_{ sp } = 1.5\times { 10 }^{ -16 } $$ Less soluble \(\longrightarrow \) Precipitate first $$ KCl + AgN{ O }_{ 3 } \rightleftharpoons AgCl \downarrow + KN{ O }_{ 3 } $$ $$ { K }_{ sp } = 1.56\times { 10 }^{ -10 } $$ More soluble \(\longrightarrow \) Precipitate Last

(iii) In qualitative analysis

Mixture of salts are analysed by precipitating out less soluble component first and more soluble components last. All the cations are classified into 6 groups based on their solubility product, first group cations are precipitated as chlorides (e.g. AgCl), while second group cations are precipitated as sulphides (e.g. \(ZnS{ O }_{ 4 }\)) and third group cations are precipitated as hydroxides (e.g. \({ Al }_{ 2 }{ (OH) }_{ 3 }\)).

(iv) Purification of NaCl

Sodium chloride from aqueous solution is precipitated by passing HCl gas, which increases ionic product \(\left[ { Na }^{ + } \right] \left[ { Cl }^{ - } \right] \) above solubility product and NaCl precipitates out.