Common-Ion Effect in Solubility Equilibria

In Examples 18-2 and 18-3, the ions in the saturated solutions came from a single source, the pure solid solute. Suppose that to the saturated solution of Pbl2 in Example 18-3, we add some 1 —a common ion—from a source such as Kl(aq). The situation is similar to our first encounter with the common-ion effect in Chapter 17. 

According to Le Chatelier s principle, an equilibrium mixture responds to a forced increase in the concentration of one of its reactants by shifting in the direction in which that reactant is consumed. In the lead(II) iodide solubility 

The addition of the common ion shifts the equilibrium of a slightly soluble ionic compound toward the undissolved compound, causing more to precipitate. Thus, the solubility of the compound is reduced. 

The solubility of a slightly soluble ionic compound is lowered in the presence of a second solute that furnishes a common ion. 

The solubility of PW2 in the presence of 0.10 MF, as calculated in Example 18-4, is about 2000 times less than its value in pure water (Example 18-3). If you work out Practice Example 18-4A, you will see that the effect of added Pb in reducing the solubility of Pbl2 is not as striking as that of F, but it is significant nevertheless. 

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