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**Colligative Properties of Solutions**

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**Objectives Describe the colligative properties of a solution.**

Calculate freezing point depression and boiling point elevation for solutions

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**Colligative Properties**

A property that depends on the concentration of solute particles but not on the identity of the solute.

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**Vapor Pressure Lowering**

As the concentration of a solute increases, the concentration of the solvent particles at the surface of a liquid decreases. This causes the vapor pressure to be lower. A 1.0 m solution of glucose (C6H12O6) lowers the vapor pressure to the same extent as a 1.0 m solution of sucrose (C12H22O11)

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**Freezing Point Depression**

Freezing Point Depression (Δtf): the difference between the freezing point of a pure solvent and a solution of a solute in the same solvent. Δtf = Kfm Kf = molal freezing point constant in oC/m m = molality

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Sample Problem What is the freezing point depression of water in a solution of 17.1 g of sucrose (C12H22O11) and 200. g of water? What is the actual freezing point of water. Water has a Molal F.P. constant of oC/m.

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Sample Problem A water solution containing an unknown quantity of a non-electrolyte solute is found to have a freezing point of oC. What is the molal concentration of the solution?

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Sample Problems Determine the freezing point of a water solution of fructose (C6H12O6) made by dissolving 58.0 g of fructose in 185 g of water. -3.24oC Calculate the molality of a solution of 39.2 g of urea, H2NCONH2, in 485 g of pure acetic acid. Determine the freezing point of this solution. For acetic acid: Normal f.p. = 16.6oC Kf = oC/m 1.35 m, 11.3 oC

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Sample Problems What is the expected change in the freezing point of water in a solution of 62.5 g of barium nitrate, Ba(NO3)2, in 1.00 kg of water.

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Sample Problems What is the expected freezing point depression for a solution that contains 2.0 mol of magnesium sulfate dissolved in 1.0 kg of water? -7.4oC A water solution contains 42.9 g of calcium nitrate dissolved in 500. g of water. Calculate the freezing point of the solution. -2.92oC

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**Boiling Point Elevation**

Boiling Point Elevation (Δtb): the difference between the boiling point of a pure solvent and a solution of a solute in the same solvent. Δtb = Kbm Kf = molal boiling point constant in oC/m m = molality

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Sample Problem A solution contains 50.0 g of sucrose, C12H22O11, a non-electrolyte, dissolved in g of water. What is the boiling point elevation?

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Sample Problems What is the boiling point of a solution of 25.0 g of 2-butoxyethanol, HOCH2CH2OC4H9, in 68.7 g of ether? (For ether, ∆tb= 2.02oC/m) 40.8oC What mass of glycerol, CH2OHCHOHCH2OH, must be dissolved in 1.00 x 103 g of water in order to have a boiling point of 104.5oC? 810g What is the expected boiling point of a 1.70 m solution of sodium sulfate in water? 102.6oC

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Ions in Aqueous Solutions

Ions in Aqueous Solutions

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