Colligative Properties
What are colligative properties?
• “Colligative” means properties that depend
only on the number of molecules in a
solution.
• Colligative properties depend on the
concentration of the solution only.
What are some colligative
properties?
• Some colligative properties are:
– Boiling point elevation
– Freezing point depression
– Osmosis
Boiling Point Elevation
• Every liquid has a molal boiling point
elevation constant.
• This number describes how much the
boiling point of the liquid will increase for
each mole of any chemical added.
Sample Boiling Point Elevation
Problem 1
• Water has a molal boiling point elevation
constant of 0.52°C and boils at 100°C.
What would the boiling point of a 3 m
solution be?
Sample Boiling Point Elevation
Problem 1
• Step One: Multiply the molal boiling point
elevation constant by the number of
moles.
• 0.52°C x 3 = 1.56°C
• Step Two: Add the answer from step one
to the boiling point of the liquid.
• 100°C + 1.56°C = 101.56°C
Sample Boiling Point Elevation Two
• Benzene boils at 80.1°C with a molal
boiling point elevation constant of 2.53°C.
What would the boiling point of a 5m
solution be?
Sample Problem Two
• Step One:
2.53°C x 5 = 12.65°C
• Step Two:
80.1°C + 12.65°C = 92.75°C
Freezing Point Depression
• Every liquid has a molal freezing point
depression constant.
• This number describes how much one
mole of any chemical will lower the
freezing point of that solution.
Freezing Point Depression Problem
One:
• Water freezes at 0°C with a molal freezing
point depression of 1.86°C. What is the
freezing point of a 2 m solution?
Freezing Point Depression Problem
One:
• Step One: Multiply the molal freezing point
depression constant by the concentration
in m
1.86°C x 2 m = 3.72°C
• Step Two: Subtract the answer from step
one from the freezing point.
0°C – 3.72°C = -3.72°C