L16 Demo Calculations-updated

L16 Demo Calculations-updated - Version 10/07/2007 The...

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Version 10/07/2007 1 The Pb(NO 3 ) 2 (aq) + 2 KI (aq) Demo from Lecture 16 Overview: Part 1 : In lecture, we added a solution of potassium iodide to a diluted solution of lead nitrate, Pb(NO 3 ) 2 . As expected from the qualitative solubilities ("soluble" and "insoluble") in Table 5-1 in your Moore et al. textbook, a precipitate of PbI 2 (s) formed. Since a stoichiometric amount of I (aq) was added to the solution of Pb +2 (i.e., in a ratio of 2:1, which is calculated below), do you expect there to be any Pb +2 in solution at equilibrium? [This question is important since it would be illegal to dump any solution containing Pb +2 into the sink since Pb +2 is toxic. Yet it is also expensive to dispose of hazardous waste, so if there is essentially no Pb +2 left in solution then the solution should not be treated as hazardous waste!]. Note that K sp = [Pb +2 ][I ] 2 = 8.7 x 10 9 . The calculations to answer this question are provided below. Part 2 : To test whether any Pb +2 is still in the solution we made in Part 1, we then filtered out the yellow precipitate with filter paper and collected the clear solution in another test tube. We then added more KI solution. A large amount of yellow precipitate was formed again! The calculations for Part 1 below show how much Pb +2 was in solution at equilibrium in the presence of PbI 2 (s) but without an excess of I , while the calculations for Part 2 below show how much Pb +2 was in solution at equilibrium in the presence of PbI 2 (s) and an excess of I . The fact that the Pb +2 concentration at equilibrium is > 2 orders of magnitude smaller in the presence of an excess of I (i.e., >2 orders of magnitude smaller in Part 2 than in Part 1) is called the COMMON ION EFFECT . In other words, by adding an excess of either the cation or anion that can form an ionic solid from the anion and cation combined, the equilibrium concentration of the *other* ion in solution can be driven to very small values (since the value of the solubility
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This note was uploaded on 04/03/2008 for the course CHEM 1A taught by Professor Nitsche during the Fall '08 term at University of California, Berkeley.

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L16 Demo Calculations-updated - Version 10/07/2007 The...

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