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Unformatted text preview: e Q value to the individual Ksp values: CaF2: Q>Ksp, therefore equilibrium will shift left and a precipitate forms. FeF2: Q<Ksp, therefore equilibrium will shift right and no precipitate forms. PbF2: Q>Ksp, therefore equilibrium will shift left and a precipitate forms. 19 Dr. Fus CHEM 123 66. A solution contains 0.010 M Al3+ and 0.010 M Ag+. Solid Na3PO4 is slowly added to separate the two cations. Ksp for AlPO4 is 1.3 x 10‐20 and Ksp for Ag3PO4 is 1.3 x 10‐20. Which cation would precipitate first, and after it precipitates, what concentration of PO43‐ ion should be obtained in the solution for the best separation? Al3+(aq) + PO43‐(aq) Q=[Al3+][PO43‐] 1.3 x 10‐20=(0.010 M)[PO43‐] [PO43‐]=1.3 x 10‐18 M needed for AlPO4 to ppt. AlPO4(s) 3Ag+(aq) + PO43‐(aq) Ag3PO4(s) +]3[PO 3‐] Q=[Ag
1.3 x 10‐20=(0.010 M)3[PO43‐] [PO43‐]=1.3 x 10‐14 M needed for Ag3PO4to ppt. *Since the concentration of PO43 needed for AlPO4 to precipitate is lower, it will precipitate first. To ensure the best separation, [PO43] should be as close to the higher concentration (1.3 x 1014 M) WITHOUT going over that concentration. This ensures that virtually all of AlPO4 will be solid, but no Ag+ will have precipitated yet into Ag3PO4. 67. A solution of NaF is added dropwise to a solution that is 0.0144 M in Ba2+. BaF2 (Ksp = 1.7 x 10‐6) will begin to precipitate when the concentration of F‐ ions reaches what value? Neglect volume changes associated with the addition of NaF solution. BaF2(s) Ba2+(aq) + 2F‐(aq) Q=[Ba2+][F‐]2 1.7 x 10‐6=(0.0144 M)[ F‐]2 [F‐]=0.011 M needed for BaF2 to ppt. 68. AgNO3 is slowly added to a solution containing the following anions: 0.02 M CO32‐ 0.02 M AsO42‐ 0.02 M I‐ ‐22
In what order will these salts precipitate? Ksp for Ag3AsO4 = 1.0 x 10 AgI = 8.3 x 10‐17 Ag2CO3 = 8.1 x 10‐12 Ag3AsO4(s) 3Ag+(aq) + AsO43‐(aq) Q=[Ag+]3[AsO43‐] 1.0 x 10‐22=[Ag+]3(0.02 M) [Ag+]=1.7 x 10‐7 M needed for Ag3AsO4 to ppt. Ag+(aq) + I‐(aq) Q=[Ag+][I‐] 8.3 x 10‐17=[Ag+](0.02 M) +]=4.2 x 10‐15 M needed for AgI to ppt. [Ag
AgI(s) 2Ag+(aq) + CO32‐(aq) Ag2CO3(s) Q=[Ag+]2[CO32‐] 8.1 x 10‐12=[Ag+]2(0.02 M) [Ag+]=2.0 x 10‐5 M needed for Ag2CO3 to ppt. *Lowest concentration will precipitate first: AgI precipitates first, then Ag3AsO4, then Ag2CO3. 20 Dr. Fus CHEM 123 69. A solution contains three anions with the following concentrations, 0.20 M CrO42‐, 0.10 M CO32‐ and 0.01 M Cl‐. If a dilute AgNO3 solution is slowly added to the solution what precipitate forms first? Ag2CrO4 (Ksp = 1.210‐12), Ag2CO3 (Ksp = 8.110‐12), AgCl (Ksp = 1.8 10‐10)? 2Ag+(aq) + CrO42‐(aq) Ag2CrO4(s) Q=[Ag+]2[CrO42‐] 1.2 x 10‐12=[Ag+]2(0.20 M) [Ag+]=2.45 x 10‐6 M needed for Ag2CrO4 to ppt. 2Ag+(aq) + CO32‐(aq) Ag2CO3(s) Q=[Ag+]2[CO32‐] 8.1 x 10‐12=[Ag+]2(0.10 M) [Ag+]=9.0 x 10‐6 M needed for Ag2CO3 to ppt. Ag+(aq) + Cl‐(aq) Q=[Ag+][Cl‐] 1.8 x 10‐10=[Ag+](0.01 M) [Ag+]=1.8 x 10‐8 M neede...
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- Spring '06