Solutions
Colligative
Properties of
Solutions
Objectives
1. 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/m
m = molality
Sample Problem
1. 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 Problem
2. 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 Problems
1. 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. = 16.6oC
Kf = -3.90 oC/m
1.35 m, 11.3 oC
Sample Problems
1. 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 Problems
1. 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/m
m = molality
Sample Problem
A solution contains 50.0 g of sucrose, C12H22O11, a nonelectrolyte, dissolved in 500.0 g of water. What is the
boiling point elevation?
Sample Problems
1. 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