solubility product

Aug 26, 2022, 16:45 IST

About Solubility Product

Here we discuss the solubilities of ionic solutes. When a salt is dissolved in water, it dissolves as the salt, AgCl (s) = AgCl(aq) and then the dissolved salt dissociates to give the ions AgCl (aq) -->Ag+ (aq) + Cl–(aq). After a certain amount of the salt has been dissolved, the solution would become saturated with the salt. Now, if some more of the salt is dissolved, the  salt would dissolve to give ions while at the same time the ions would precipitate to give back the salt. This amount (moles) of the salt that has made the solution saturated per liter of solution is called the solubility of the salt.

Application of solubility product

a)  Predicting the formation of ppt.:  A solution of   is added to a solution of 107 M CaCl2.  If Ksp for AgCl is 1.7  1010, predict whether precipitate will form.

b)  Predicting solubility of sparingly soluble salts:

c) Purification of common salt :  This is achieved by passing dry HCl gas through brine (conc. NaCl) solution.  Since excess Cl are now available, [H+]. [Cl] i.e., ionic product becomes greater than Ksp; NaCl precipitates out and can be filtered off from the solution.

d) Salting out of soap:  Soaps are Na or K salts of higher fatty acids.  They can be made to ppt by adding NaCl or KCl.

e)  Inorganic analysis:

The complete analysis of basic radicals excluding zero group (NH4+) is based upon solubility product.

i)  1st group:  Pb2+, Ag+ and Hg22+ are precipitated in this group as chlorides.  Ksp of the chlorides of above ions are comparatively lower than the other radicals.  So when dil HCl is added, sufficient Cl are produced to precipitate these radicals, while those of all the other radicals/ions being highly soluble (with much higher Ksp values) remain in the solution in dissolved state.

ii)  IInd group: IInd group and IV group both are precipitated as sulphides. But to limit the precipitation of only IInd group and not those of Vth or any higher group (III, V, VI etc) we make use of their solubility products.  For IInd group radicals HCl is used along with H2S, so as to produce only limited number of S2.

iii)  IIIrd group: Al3+, Fe3+ & Cr3+ are precipitated as hydroxides in presence of NH4OH & NH4Cl.  They are selectively precipitated as hydroxides using NH4OH + NH4Cl (called group reagent) ; since more OH may be produced in the solution otherwise which can precipitate even IV or higher group radicals/ions.  Their concentration is controlled (decreased) by using common ion effect, permitting to precipitate only IIIrd group hydroxides.

Example of solubility product: