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