Page 102 - Instant notes

Physical chemistry 88

−
+
where and are the activity coefficients of M and X and is the standard
−3
concentration of 1 mol dm . For sparingly soluble salts, such as silver chloride, which
−3
have concentrations much less than 0.001 mol dm , there is negligible interaction
between ions in solution and the activity coefficients can be approximated to unity. The
equation then becomes:

The solubility, s, of the salt is the concentration of dissolved salt in the solution. For a
−
+
salt MX, as one mole of M and X ions is produced by the dissolution of
one mole of salt. Therefore:

which allows the solubility of the salt in water to be determined from K sp. For sparingly
soluble salts containing ions with differing stoichiometries, a similar expression can be
obtained. For example for silver sulfide, Ag 2S, the solubility equilibrium is:

One mole of silver sulfide dissolves to form one mole of sulfide ions and two moles of
silver ions.

The common ion effect
The common ion effect considers the effect on the solubility of the salt MX of adding
−
+
either M or X . An example is the addition of NaCl to a saturated AgCl solution. Le
Chatelier’s principle predicts that the equilibrium:

will shift to the left to counteract the increase in chloride ion concentration and that the
solubility will decrease. Quantitatively, if a concentration, c, of NaCl is added which is
−
enough to swamp the original concentration of Cl in solution, then . The