Solutions

Colligative Properties of

Solutions

Objectives1. Describe the colligative properties of a

solution.2. Calculate freezing point depression and

boiling point elevation for solutions

Colligative Properties

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

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)

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/mm = molality

Sample Problem1. 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 -1.86 oC/m.

Sample Problem2. A water solution containing an unknown quantity of

a non-electrolyte solute is found to have a freezing point of -0.23 oC. What is the molal concentration of the solution?

Sample Problems1. 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

2. 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. = 16.6oCKf = -3.90 oC/m1.35 m, 11.3 oC

Sample Problems1. 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.

Sample Problems1. 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

2. 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

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/mm = molality

Sample ProblemA solution contains 50.0 g of sucrose, C12H22O11, a non-

electrolyte, dissolved in 500.0 g of water. What is the boiling point elevation?

Sample Problems1. 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

2. 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

3. What is the expected boiling point of a 1.70 m solution of sodium sulfate in water?102.6oC