A More Complex Problem The Common Ion Effect

Calculating the solubility of Pb(I03)2 in distilled water is a straightforward problem since the dissolution of the solid is the only source of Pb + or lOa. How is the solubility of Pb(I03)2 affected if we add Pb(I03)2 to a solution of 0.10 M Pb(N03)2 Before we set up and solve the problem algebraically, think about the chemistry occurring in this system, and decide whether the solubility of Pb(I03)2 will increase, decrease, or remain the same. This is a good habit to develop. Knowing what answers are reasonable will help you spot errors in your calculations and give you more confidence that your solution to a problem is correct. 

We begin by setting up a table to help us keep track of the concentrations of Pb + and 103 in this system. 

Substituting the equilibrium concentrations into the solubility product expression (equation 6.33) 

This is a more difficult equation to solve than that for the solubility of Pb(I03)2 in distilled water, and its solution is not immediately obvious. A rigorous solution to equation 6.34 can be found using available computer software packages and spreadsheets. 

How might we solve equation 6.34 if we do not have access to a computer One possibility is that we can apply our understanding of chemistry to simpKfy the algebra. From Le Chatelier s principle, we expect that the large initial concentration of Pb will significantly decrease the solubility of Pb(I03)2. In this case we can reasonably expect the equiKbrium concentration of Pb to be very close to its initial concentration thus, the following approximation for the equilibrium concentration of Pb seems reasonable 

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