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## Colligative Properties

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### Colligative Properties Chapter 13, Section 2 * What is a colligative property? A property that depends on the concentration* of solute particles NOT the identity ... – PowerPoint PPT presentation

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Title: Colligative Properties

1
Colligative Properties
• Chapter 13, Section 2

2
What is a colligative property?
• A property that depends on the concentration of
solute particles NOT the identity of the
particle.
• Vapor Pressure Reduction
• Boiling Point Elevation
• Freezing Point Depression
• Osmotic Pressure
• Concentration will be in terms of molality (m)

3
• Vapor pressure is pressure caused by molecules in
the gas phase that are in equilibrium with the
liquid phase. (in a closed system particles go
back and forth between phases)

4
Vapor Pressure Reduction
• The pressure of the vapor over a solvent is
reduced when a solute is dissolved in it. Vapor
pressure reduction is directly related to
concentration of the solution.

5
• Why does this occur?
• Increasing solute particles reduces the
proportion of solute to solvent. Fewer solvent
particles will be available to leave the solution
and enter the gas phase.
• The solution will remain a liquid over a larger
temp. range.

6
Phase Diagram
Shows us what phase the solvent will be in at a
given temperature and pressure.
If we add a solute that lowers the vapor
pressure, then our phase diagram adjusts
7
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8
Boiling Point Elevation
• The boiling point of a solvent is raised when a
solute is dissolved in it. It is directly
related to the concentration of the solution.
• Boiling point the temp. at which vapor pressure
of a liquid is equal to atmospheric pressure

9
• The change in boiling point is proportional to
the molality of the solution
• ?Tb Kb ? m
• Kb is the molal boiling point elevation
constant, a property of the solvent. It has the
units ºC/m.
• ?Tb is the boiling point elevation. It is the
difference between the boiling points of the pure
solvent and that of the solution.

10
Freezing Point Depression
• The freezing point of a solvent is lowered when a
solute is dissolved in it. Freezing point
depression is directly related to concentration
of a solution.
• Freezing point the temp. where vapor pressure
of the solid and liquid phases are the same.

11
• The change in freezing point can be found
similarly
• ?Tf Kf ? m
• Here Kf is the molal freezing point depression
constant of the solvent.

12
Osmotic Pressure (?)
• The pressure required to prevent osmosis.
• Osmosis is the flow of particles from a dilute
solution to a concentrated one across a semi
permeable membrane.

13
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14
Electrolytes change moles of solute
• Because ionic compounds dissociate in water, they
will increase the concentration of dissolved
particles.
• When determining or using moles of an
electrolyte, use the dissociated amount.
• AlBr3 (s) ? Al3 (aq) 3 Br- (aq)

15
• If 50.0 g of BaCl2 are added to 2.5 kg of
water, compute the change in freezing point and
state the new freezing point of the solution.
• Water Kf -1.86?C/m Kb.51 ?C/m

16
Drill 4/29/11
• Write everything you know about acids and bases

17
• Determine the freezing point of a solution of
60.0g of glucose, C6H12O6, dissolved in 80.0g of
water.

18
• Workbook pg. 226 1, pg. 229 1, pg. 232 1
• (use the table on pg. 223 for Kb or Kf)
• Then pg. 233 3, 11 and pg. 234 17