Applications of Solubility Product
(i) In predicting the formation of a precipitate
Case I: When, then solution is unsaturated in which more solute can be dissolved. i.e., no precipitation.
Case II: When , then solution is saturated in which no more solute can be dissolved but no ppt. is fomed.
Case III: When , then solution is supersaturated and precipitation takes place.
When the ionic product exceeds the solubility product, the equilibrium shifts towards left-hand side, i.e., increasing the concentration of undissociated molecules of the electrolyte. As the solvent can hold a fixed amount of electrolyte at a definite temperature, the excess of the electrolyte is thrown out from the solutions as precipitate.
(ii) In predicting the solubility of sparingly soluble salts Knowing the solubility product of a sparingly soluble salt at any given temperature, we can predict its solubility.
(iii) Purification of common salt: gas is circulated through the saturated solution of common salt. and dissociate into their respective ions as,
⇌ ; ⇌
The concentration of ions increases considerably in solution due to ionisation of and due to common ion effect, dissociation of NaCl is decreased. Hence, the ionic product exceeds the solubility product of and therefore pure precipitates out from the solution.
(iv) Salting out of soap: From the solution, soap is precipitated by the addition of concentrated solution of .
⇌ ; ⇌
Hence, the ionic product [RCOO–] [Na+] exceeds the solubility product of soap and therefore, soap precipitates out from the solution.
(v) In qualitative analysis: The separation and identification of various basic radicals into different groups is based upon solubility product principle and common ion effect.
(a) Precipitation of group first radicals(Pb+2, Ag+ , Hg+2)The group reagent is dilute HCl. for AgCl.
(b) Precipitation of group second radicals(Hg+2, Pb+2, Bi+3, Cu+2, Cd+2, As+3, Sb+3 and Sn+2):The group reagent is in presence of dilute . for .
(c) Precipitation of group third radicals(Fe+3, Al+3 and Cr+3)The group reagent is in presence of .
(d) Precipitation of group fourth radicals(Co+2, Ni+2, Mn+2 and Zn+2) : The group reagent is in presence of .
(e) Precipitation of group fifth radicals (Ba+2, Sr+2, Ca+2) The group reagent is ammonium carbonate in presence of and .
(vi) Calculation of remaining concentration after precipitation: Sometimes an ion remains after precipitation if it is in excess. Remaining concentration can be determined,
% precipitation of ion =
(vii) Calculation of simultaneous solubility : Solubility of two electrolytes having common ion; when they are dissolved in the same solution, is called simultaneous solubility.
Calculation of simultaneous solubility is divided into two cases.
Case I: When the two electrolytes are almost equally strong (having close solubility product).
Here, charge balancing concept is applied.
Charge of = Charge of + Charge of
Case II: When solubility products of two electrolytes are not close, i.e., they are not equally strong.
Most of fluoride ions come of stronger electrolyte.