Solubility and pH

Learning Goals

You will learn how and why the solubility of salts can be affected by the pH of the solution.

Synopsis

Although some salts may be extremely insoluble in water, the addition of an acid or a base may drastically change the solubility properties of the salt. The PbS solubility is a good example of this process that could involve several simultaneous equilibrium reactions. The reaction PbS(s) Pb2+(aq) + S2-(aq) has Ksp = 3.2 x 10-28, but the S2- ion is extensively hydrolyzed: S2-(aq) + H2O(l) HS-(aq) + OH-(aq) which has Kb = 1 x 105. The overall process has an equilibrium constant that is the product of these two constants, 3.2 x 10-23. Thus the sulfide is more soluble in water than indicated by its single equilibrium constant. Moreover, if we add an acid to the solution we will be removing some of the hydroxyl ions which [Le Chatelier principle] will shift the hydrolysis reaction further toward the products. Shifting the reaction toward the products will require more S2- ions, therefore increasing the solubility of the sulfide.

In general, the solubility of a salt containing the conjugate base of a weak acid is increased by the addition of a stronger acid to the solution. However, if the anion of a salt is the conjugate base of a strong acid the salt is not soluble in the strong acid.

Review Questions

  1. Will the solubility of FeS be increased by the addition of a strong acid?
  2. Will the solubility of AgCl be increased by the addition of a strong acid?


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Web Author: Dr. Leon L. Combs
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