Ions and the Solubility of Precipitates

In Section 19.3, you saw that H3O increases the solubility of a slightly soluble ionic compound if its anion is that of a weak acid. Similarly, a ligand increases the solubility of a slightly soluble ionic compound if it forms a complex ion with the cation. For example, zinc sulfide is very slightly soluble  

When we add some 1.0 M NaCN, the CN ions act as ligands and react with the small amount of Zn aq) to form the complex ion Zn(CN)4  

To see the effect of complex-ion formation on the solubility of ZnS, we add the equations and, therefore, multiply their equilibrium constants  

The overall equilibrium constant increased by more than a factor of 10 in the presence of the ligand this reflects the increased amount of ZnS in solution. 

SAMPLE PROBLEM 19.10 Calculating the Effect of Complex-Ion Formation 

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