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Solubility Product Constant Factors Affecting Solubility Selective Precipitation

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Objectives ▫Today I will be able to: Calculate the solubility product constant for a compound Explain the factors that affect solubility Determine the compounds that selectively precipitate from a solution when placed under stresses Informal assessment – monitoring student interactions and questions as they complete the practice problems Formal assessment – analyzing student responses to the practice problems

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Lesson Sequence Evaluate: Warm Up Explain: Notes Elaborate: Practice Problems Evaluate: Exit Ticket

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Warm Up

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Objectives Today I will be able to: ▫Calculate the solubility product constant for a compound ▫Explain the factors that affect solubility ▫Determine the compounds that selectively precipitate from a solution when placed under stresses

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Homework Continue working practice problems Ch. 17 49, 51,53, 55, 57,59 69,70, 75,76

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Agenda Warm Up Notes Practice Problems Exit Ticket

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Solubility Product Constant Factors Affecting Solubility Selective Precipitation Notes

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Solubility Equilibria

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Solubility Product Constant Consider the equilibrium that exists in a saturated solution of BaSO 4 in water: BaSO 4 (s) Ba 2+ (aq) + SO 4 2− (aq)

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Solubility Product Constant Expression The equilibrium constant expression for this equilibrium is K sp = [Ba 2+ ] [SO 4 2− ] where the equilibrium constant, K sp, is called the solubility product.

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Ksp is not the same as solubility

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What is solubility?What is Ksp? Solubility is generally expressed as the mass of solute dissolved in 1 L (g/L) or 100 mL (g/mL) of solution, or in mol/L (M). Equilibrium constant for equilibrium between an ionic sold and its saturated solution Unitless Larger magnitude means more dissolving occurred Only one value per temperature Does not change with pH or other ions present in solution like solubility

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Calculation Solid silver chromate is added to pure water at 25 o C and some solid remains undissolved. The mixture is stirred for several days to ensure that equilibrium is achieved between the undissolved Ag 2 CrO 4 and the solution. Analysis of the equilibrated solution shows that its silver ion concentration is 1.3 x 10 -4 M. Assuming that the Ag 2 CrO 4 solution is saturated, calculate Ksp for the compound.

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Calculation The K sp of CaF 2 is 3.9 x 1o -11 at 25 o C. Assuming equilibrium is established between solid and dissolved CaF 2, calculate the solubility of CaF 2 in grams/liter

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Factors impacting solubility

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1. Common Ion Effect If one of the ions in a solution equilibrium is already dissolved in the solution, the equilibrium will shift to the left and the solubility of the salt will decrease BaSO 4 (s) Ba 2+ (aq) + SO 4 2− (aq)

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2. pH ▫If a substance has a basic anion, it will be more soluble in an acidic solution. ▫Substances with acidic cations are more soluble in basic solutions.

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3. Complex Ions Complex Ions ▫Metal ions can act as Lewis acids and form complex ions with Lewis bases in the solvent.

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4. Amphoterism ▫Amphoteric metal oxides and hydroxides are soluble in strong acid or base, because they can act either as acids or bases. ▫Examples of such cations are Al 3+, Zn 2+, and Sn 2+

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Complex Ions Cont. The formation of these complex ions increases the solubility of these salts.

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Precipitation and Separation of Ions

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Will a Precipitate Form? In a solution, ▫If Q = K sp, the system is at equilibrium and the solution is saturated. ▫If Q < K sp, more solid will dissolve until Q = K sp. ▫If Q > K sp, the salt will precipitate until Q = K sp.

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Sample Calculation The K sp for BaSO 4 is 1.1 x 10¯ 10 and that for BaSeO 4 is 2.8 x 10¯ 11. A 1.0 M solution of BaCl 2 is added slowly to a solution that is 1.0 x 10¯ 4 M in sodium sulfate, Na 2 SO 4 and 1.0 x 10¯ 4 M in sodium selenate, Na 2 SeO 4. What is the approximate percentage of one anion has precipitated at the point which the second anion just begins to precipitate? (Assume the addition of the BaCl 2 solution does not change the overall solution volume to any significant degree.)

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Solution

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Selective Precipitation of Ions One can use differences in solubilities of salts to separate ions in a mixture.

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Selective Precipitation Practice Problems Visit the website and walk through the sample calculations Try to solve the problem on your own and then walk through the worked out solution http://www.chemteam.info/Equilibrium/Selecti vePrecip.html

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Practice Problems Ch. 17 49, 51,53, 55, 57,59 69,70, 75,76

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Closure Which problem did you find most challenging today?

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