Mastering Chemistry Help

Qualitative analysis of cations is largely based on the principle of solubility product and common ion effect. Cations are separated in to six groups depending on the solubility of their salts. Group-1 as insoluble chlorides Only Ag + , Hg 2+ and Pb 2+ form insoluble chlorides since they have low values of K sp . Group-2 as insoluble sulphide in acidic medium H 2 S <========> H + + HS - ; K 1 - first ionization constant HS - <========> H + + S 2 - ; K 2 – second ionization constant [S 2 - ] = K 1 K 2 [H 2 S]/[H + ] 2 K sp values of second group sulphides (PbS, CuS, SnS, HgS, As 2 S 3 , Bi 2 S 3 , Sb 2 S 3 ) are very low. In acidic buffer, [S 2 - ] is decreased due to common ion effect and this results in the precipitation of Pb 2+ , Cu 2+ etc of second group as their sulphides. Third and fourth group sulphides have high value of K sp , hence they remain soluble. Group- 3 as insoluble hydroxide in basic buffer of NH 4 OH and NH 4 Cl The concentration of OH - in a...

The decrease in the ionization of a weak electrolyte by the presence of a common-ion from a strong electrolyte, is called the common ion effect . Ionisation of CH 3 COOH (weak acid) is decreased by the addition of CH 3 COONa (CH 3 COO- being the common ion) CH 3 COOH CH 3 COO - + H + ……………………….. (A) CH 3 COONa -----------> CH 3 COO - + Na + In the presence of CH 3 COO - equilibrium (A) shifts in backward direction. Ionisation of H 2 S (weak acid) is decreased by the addition of HCl (H + being the common ion) H 2 S 2H + + s 2- HCl H + + Cl - Ionisation of NH 4 OH (weak base) is decreased by the addition of NH 4 Cl (NH4 + being the common ion) NH 4 OH NH 4 + + OH - NH 4 Cl --------------> NH 4 + + Cl - Solubility of a sparingly soluble salt is decreased by the addition of common ion. Presence of AgNO 3 or KCl decreases the solubility of AgCl. AgCl Ag + + Cl - AgNO 3 Ag + (common ion) + NO 3 - KCl K + + Cl - (common ion) The common ion effect is thus ba...