In accordance with Le-Chatelier’s Principle if we increase the concentration of one of the ions, in equilibrium with the solid salt, it should combine with the ion of its opposite charge and some of the salt will be precipitated. Precipitation will continue till Qs becomes equal to K sp · Similarly, if the concentration of one of the ions is decreased, Q sp becomes less than K sp · Some of the salt dissolves, to increase the concentrations of both the ions so that again Q sp becomes equal to K sp · The common ion effect finds application in the purification Of sodium chloride and in_ the precipitation of soap. Pure sodium chloride is precipitated by passing HCl gas through a saturated solution of impure sodium chloride
Due to increase in concentration of Cl- ions the equilibrium in equation (28.3). Shifts in backward direction resulting in the precipitation of pure sodium chloride. Impurities present in the sodium chloride remain in the solution.
CALCULATION OF SOLUBILITY
If solubility product of a sparingly soluble salt at a particular temperature is known, its solubility at that temperature can be calculated.
Expressions, correlating K sp and solubility (mol L-1) of some common types of salts are listed below: