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Chemistry 1062: Principles of Chemistry II Monday, April 11, 2005

Chemistry 1062: Principles of Chemistry II Monday, April 11, 2005. Instructor: Lance S. Lund. Predicting if Precipitation Occurs. Crystals of calcium oxalate may form in the kidneys if the concentration of the oxalate ion becomes too large.

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Chemistry 1062: Principles of Chemistry II Monday, April 11, 2005

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  1. Chemistry 1062: Principles of Chemistry IIMonday, April 11, 2005 Instructor: Lance S. Lund

  2. 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, [Ca2+] = 0.0025 M and [C2O42-] = 1.0 x 10-7M. Do you expect calcium oxalate to precipitate under these conditions?

  3. 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 Qsp < Ksp, then reaction shifts right. • If Qsp > Ksp, then reaction shifts left. • If Qsp = Ksp, then reaction is at equilibrium.

  4. 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 Pb2+ ion in a precipitation or if less than 0.01% of the Hg2+ ion remains in a different procedure.

  5. Precipitation Problem • Lead chromate, PbCrO4, is a yellow pigment used in paints. Suppose 0.50 L of 1.0 x 10-5M Pb(C2H3O2)2 and 0.50 L of 1.0 x 10-3M K2CrO4 are mixed. • Calculate [Pb2+] (at equilibrium) after PbCrO4 precipitates. • What is the percentage of Pb2+ remaining in the solution after PbCrO4 precipitates?

  6. Precipitation Problem Answers • [Pb2+] = 3.6 x 10-11M • 0.00073% of Pb2+ remains

  7. Fractional Precipitation • 1.0 M AgNO3 is slowly added to a solution containing 0.015 M Cl- and 0.015 M CrO42- • Which anion precipitates first? • What is the concentration of the first anion in solution just as the second anion begins to precipitate from solution?

  8. Fractional Precipitation • Cl- precipitates first from the solution. • The [Cl-] just as Ag2CrO4 begins to precipitate is 2.1 x 10-5M.

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