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Chemistry 1062: Principles of Chemistry II Monday, April 11, 2005 Instructor: Lance S. Lund

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Predicting if Precipitation Occurs Crystals of calcium oxalate may form in the kidneys if the concentration of the oxalate ion becomes too large. In a blood plasma sample, [Ca 2+ ] = 0.0025 M and [C 2 O 4 2- ] = 1.0 x 10 -7 M. Do you expect calcium oxalate to precipitate under these conditions?

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Criterion for Precipitation Recall that in order to determine whether or not precipitation will occur, calculate the reaction quotient, Q, and compare it to the equilibrium constant, K. If Q sp < K sp, then reaction shifts right. If Q sp > K sp, then reaction shifts left. If Q sp = K sp, then reaction is at equilibrium.

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Completeness of Precipitation In some cases, you may want to know how much of a particular ion remains in solution or how much of a particular ion is removed. Perhaps you want to know if a procedure removes at least 99.9% of the Pb 2+ ion in a precipitation or if less than 0.01% of the Hg 2+ ion remains in a different procedure.

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Precipitation Problem Lead chromate, PbCrO 4, is a yellow pigment used in paints. Suppose 0.50 L of 1.0 x 10 -5 M Pb(C 2 H 3 O 2 ) 2 and 0.50 L of 1.0 x 10 -3 M K 2 CrO 4 are mixed. Calculate [Pb 2+ ] (at equilibrium) after PbCrO 4 precipitates. What is the percentage of Pb 2+ remaining in the solution after PbCrO 4 precipitates?

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Precipitation Problem Answers [Pb 2+ ] = 3.6 x 10 -11 M 0.00073% of Pb 2+ remains

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Fractional Precipitation 1.0 M AgNO 3 is slowly added to a solution containing 0.015 M Cl - and 0.015 M CrO 4 2- Which anion precipitates first? What is the concentration of the first anion in solution just as the second anion begins to precipitate from solution?

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Fractional Precipitation Cl - precipitates first from the solution. The [Cl - ] just as Ag 2 CrO 4 begins to precipitate is 2.1 x 10 -5 M.

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