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119_7_Solubility_06F - VI Ionic Equilibria Solubility of...

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1 VI. Ionic Equilibria Solubility of Precipitates Harris: 7 th Edition Chap. 6, 96 – 104, Ex. 6 C-F Chap. 7, 127 – 133, Ex. 7 E-G Chap. 8, Ex. 8 A-H
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2 Ionic Equilibria Solubility of Precipitates Solubilities of slightly soluble substances 10 SP EQ 10 * 1.69 ] ][Cl [Ag K [AgCl{s}] {aq)}] [Cl {aq}] [Ag K {aq} Cl {aq} Ag AgCl{s] - - + - + - + = = + 12 2 4 2 SP 4 2 2 4 2 EQ 2 4 4 2 10 1.2 ] [CrO ] [Ag K {s}] CrO [Ag {aq}] [CrO {aq}] [Ag K {aq} CrO {aq} Ag 2 {s} CrO Ag - - + - + - + = = +
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3 Ionic Equilibria Solubility of Precipitates: Ideal Solubility No activity coefficients. No complexation. No ion pair formation. No excess of either ion. No pH effects. BaCO 3 (s) Ba ++ (aq) + CO 3 = (aq) K SP (BaCO 3 )=5.0*10 –9 @25 o C (App.F, Harris) S(BaCO 3 ) = ? M S(BaCO 3 ) = mol BaCO 3 dissolved per L of solution S(BaCO 3 ) = [Ba ++ ]{1 mol BaCO 3 /1 mol Ba ++ } S(BaCO 3 ) = [CO 3 = ]{1 mol BaCO 3 /1 mol CO 3 = } K SP =[Ba ++ ][CO 3 = ] = S(BaCO 3 ) 2 S(BaCO 3 ) =√K SP = 7.1*10 -5 M S(BaCO 3 , H 2 O, 18 o ) = 1.1*10 -4 M Exp/Theo = 1.6 (CRC Handbook)
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4 Ionic Equilibria Solubility of Precipitates: Ideal Solubility K SP (CaF 2 ) = 4.9*10 –11 @ 25 o C S(CaF 2 ) = ? CaF 2 (s) Ca ++ (aq) + 2 F (aq) K SP =[Ca 2+ ][F ] 2 S(CaF 2 ) = [Ca 2+ ]{1 mol CaF 2 /1 mol Ca 2+ } [F ] = [Ca 2+ ]{2 mol F /1 mol Ca 2+ } = 2S(CaF 2 ) K SP =[Ca 2+ ][F ] = S(2*S) 2 = 4*S 3 S = 3 √( K SP /4) S(CaF 2 , water, 25 o C) = 2.3*10 –4 M = 0.018 g CaF 2 /L S(CaF 2 , water, 16 o C) = 0.016 g/L Exp/Theo = 1.1 (CRC Handbook)
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5 Solubility of Precipitates: Precipitation on Mixing 25.0 mL of 0.00155 F MgCl 2 is added to 35.0 mL of 0.00172 F NaCO 3 . K SP (MgCO 3 ) = 1.0*10 –5 . Will pptn occur? MgCO 3 (s) Mg 2+ (aq) + CO 3 = (aq) K SP = [Mg 2+ ] EQ [CO 3 = ] EQ at equilibrium Q = [Mg 2+ ][CO 3 = ] under any conditions If Q > K SP , pptn will occur. If Q < K SP , pptn will not occur. (When the two solutions are mixed, concentrations change because the volume increases: the dilution factor.)
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6 Solubility of Precipitates: Precipitation on Mixing Concentrations change on mixing whether or not reaction occurs 2 2 3 0.00155mmol Mg (25.0mL) mL [Mg ] 0.65 mM 0.65 10 M (25.0 35.0)mL + + - = = = + 2 3 2 3 3 0.00172mmolCO (25.0mL) mL [CO ] 1.00mM 1.00 10 M (25.0 35.0)mL - - - = = = +
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7 Solubility of Precipitates: Precipitation on Mixing Since Q < K SP , pptn does not occur. If initial concentrations increase by > factor of 2, pptn will occur. ( 29 ( 29 2+ 2- -3 -3 6 Exp 3 Exp [Mg ] [CO ] {=Q} = 0.65*10 M 1.00*10 M 0.65*10 - = 2+ 2- 6 5 Exp 3 Exp SP 3 [Mg ] [CO ] 0.65*10 K {MgCO } 1.0*10 - - = =
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8 Solubility of Salts: Common Ion Effect The solubility of a salt is always reduced by a (small) excess of an ion of the salt. The solubility of a salt is always less in a solution containing one of the ions of the salt than it is in pure water. MX(s) M + (aq) + X (aq) From LeChatelier’s principle, increasing [M + ] or [X ] shifts the equilibrium to the left, increases the amount of MX(s) and reduces the solubility.
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9 Solubility of Precipitates: Common Ion Effect Calculate the solubility of BaSO 4 {s} in 0.200 F Na 2 SO 4 . BaSO 4 (s) Ba 2+ {aq} + SO 4 = {aq} K SP =[Ba 2+ ][SO 4 = ] = 1.0*10 –10 Under these conditions, [Ba 2+ ] ≠ [SO 4 = ] (unlike pure water) because there is another source of SO 4 = ions!
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