Chapter 21
Colligative Properties:
Part 1
Four Colligative Properties of Solutions
Vapor pressure lowering
Boiling point elevation
Freezing point depression
Osmotic pressure
Properties determined by the number of solute
particles in solution rather than the type of particles.
What are the different properties of these solutions?
sugar and water
salt and water
How do the colligative properties of
these solutions compare?
sugar and water
salt and water
Vapor Pressure Lowering
How Vapor Pressure Lowering Occurs
• Solute particles take up space in a
solution.
• Solute particles on the surface
decrease the number of solvent
particles on the surface.
• Less solvent particles can evaporate
from the surface which lowers the
vapor pressure of the liquid.
Raoult’s Law
Vapor Pressure of solution =
(mole fractionsolvent)(Vapor Pressuresolvent)
Remember that the mole fractions in a
solution must add up to equal “1”
Vapor Pressure Table
What is the vapor pressure of an aqueous solution
at 70oC if 2.00 x 102g of sucrose (C12H22O11) are
dissolved in 1.00 x 102g of water.
28.2 kPa
What is the vapor pressure of an aqueous solution
at 70oC if 2.00 x 102g of sucrose (C12H22O11) are
dissolved in 1.00 x 102g of water.
What is the vapor pressure of an aqueous solution
at 70oC if 3.15 x 102g of C15H9NO4 are dissolved in
5.00 x 102g of water.
Answer: 29.9 kPa
What is the vapor pressure of an aqueous solution
at 70oC if 3.15 x 102g of C15H9NO4 are dissolved in
5.00 x 102g of water.
29.9 kPa
Boiling Point Elevation
• The boiling point is the temperature at which the
vapor pressure of a liquid equals the air pressure.
Boiling Point Elevation
• The boiling point is the temperature at which the
vapor pressure of a liquid equals the air pressure.
• Dissolved solute particles lower the vapor
pressure of the solvent. This requires a higher
temperature to get the vapor pressure to equal air
pressure.
Freezing Point Depression
• The freezing point is the temperature at which a
liquid crystallizes to a solid.
• Solute particles disrupt the formation of crystals
in the solvent.
• This disruption in the freezing process results in a
lowering (depression) of the freezing point.
Homework
• Worksheet: Vapor Pressure, Boiling Point and
Freezing Point.
– You should be able to complete the front of this
worksheet
– Due in two days
• Remember the “Review and Reinforcement”
Worksheet is due tomorrow.