1. Colligative Properties of Solutions

2. Objectives Describe the colligative properties of a solution.
Calculate freezing point depression and boiling point elevation for solutions

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

5. 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)

7. 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

8. 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.

9. 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?

10. 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

11. 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.

12. 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

13. 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

14. 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?

15. 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