Ch11_notes07Solub

Ch11_notes07Solub - Ch. 11 Solubility and precipitation...

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Ch. 11 Solubility and precipitation equilibria Cadmium, chromium (III), aluminum, and nickel (II) hydroxides Solubility and solubility product Effect of pH Common ion effect Complex ions and solubility
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What happens when solids are dissolved in water? Salt water
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Ionic salts (NaCl): NaCl(s) Na + + Cl - Na + (aq) + Cl - (aq) - + + - + -
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How much solute can we dissolve in 1 L of solvent? Characteristic of substance and solvent, called solubility. Solubilities of some salts in water at 25 C: 0.0018 1.3x10 -5 AgCl 5,570 27 AgClO 4 351 6 NaCl gram/L M/L Salt
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Solubility depends on temperature: Some salts are more soluble at higher temp and some less
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In saturated solution there is an equilibrium: NaCl(s) Na + (aq) + Cl - (aq) Define K for dissolving salts as K sp K sp =[Na + ][Cl - ]= x * x = x 2 = 6 2 = 36 Le Chatelier: Add water (reduce concentration): to right Evaporate water: to left Add salt: no effect Add Cl - : to left Change T: K sp will change
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L23: Solubility and precipitation equilibria Solubility : the greatest amount that will dissolve in a specified volume of a solvent at particular conditions (T, pH, presence of other ions, etc). Units: in grams or mole per liter. Solubility product : equilibrium constant (conc. in mol/L) For an ionic solid A n B m : A n B m (s) nA +x (aq) + mB -y (aq) K sp =[A +x ] n [B -y ] m [neutrality in salt must be maintained.] Example: Al 2 S 3 (s) 2Al +3 (aq) + 3S -2 (aq) K sp =[Al +3 ] 2 [S -2 ] 3
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Which compound has larger solubility : Ag 2 CrO 4 (K sp =9.0x10 -12 ) or BaCrO 4 (K sp =8.5x10 -11 )? Need to find concentration of ions in mol/L Ag 2 CrO 4 (s) 2 Ag + (aq) + CrO 4 -2 (aq) K sp = [Ag + ] 2 [CrO 4 -2 ]=9.0x10 -12 -y 2 yy K sp = (2 y ) 2 * y =4 y 3 = 9.0x10 -12 y 3 =2.25x10 -11 -> y = 2.82x10 -4 mol/L = solubility of Ag 2 CrO 4 (s) BaCrO 4 (s) <-> Ba +2 + CrO 4 -2 K sp =[Ba +2 ][CrO 4 -2 ]= 8.5x10 -11 -x -> x x K sp = x * x = x 2 =8.5x10 -11 x=9.22x10 -6 mol/L = solubility of BaCrO 4 (s) Solubility of of Ag 2 CrO 4 is larger that that of BaCrO 4 (more moles of Ag 2 CrO 4 will dissolve in the same amount of water) even though its K sp is smaller!
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K sp can be used to compare relative solubility only for compounds which produce the same total number of ions upon dissociation e.g., can compare BaSO 4 , BaCrO 4 , AgCl, CaCO 3 , FeS, ... or BaF 2 , MgF 2 , PbCl 2 , Ni(OH) 2 , Ag 2 SO 4 , Ag 2 CrO 4 , . ... but to compare compounds from different series – need to calculate solubility.
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Consider Ag 3 PO 4 (K sp =1.8x10 -18 ) and AgCl (K sp =1.6x10 -5 )? How would the solubility be affected by pH?
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This note was uploaded on 09/17/2009 for the course CHEM 115BL taught by Professor Qin,reisler during the Spring '08 term at USC.

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Ch11_notes07Solub - Ch. 11 Solubility and precipitation...

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