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Chapter Outline
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11.1 Energy Changes when Substances Dissolve
11.2 Vapor Pressure
11.3 Mixtures of Volatile Substances
11.4 Colligative Properties of Solutions
11.5 Osmosis and Osmotic Pressure
11.6 Using Colligative Properties to Determine Molar Mass
Vapor Pressure
Pressure exerted by a gas in equilibrium with its liquid
Where are we going with this?
Colligative Properties – boiling
point elevation, freezing point
depression, osmosis
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Vapor Pressure of Solutions
• Vapor Pressure:
– Pressure exerted by a gas
in equilibrium with liquid
– Rates of
evaporation/condensation
are equal
The tendency for a liquid to evaporate
increases as 1. the temperature rises
2. the surface area
increases
3. the intermolecular
forces decrease
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Boiling Point
“If you have a beaker of water open to the
atmosphere, the mass of the atmosphere is
pressing down on the surface. As heat is
added, more and more water evaporates,
pushing the molecules of the atmosphere
aside (wsystem < 0). If enough heat is added,
a temperature is eventually reached at which
the vapor pressure of the liquid equals the
atmospheric pressure, and the liquid boils.”
Normal boiling point
the temperature at which the vapor pressure of a liquid is equal to the
external atmospheric pressure of 1 atm.
Increasing the external atmospheric pressure increases the boiling point
Decreasing the external atmospheric pressure decreases the boiling point
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Location
San Francisco
Salt Lake City
Denver
Mt. Everest
Elevation (ft)
b.p H2O (oC)
sea level
4400
5280
29,028
100.0
95.6
95.0
76.5
Intermolecular Forces and Vapor Pressure
Diethyl ether - dipole-dipole
interactions
Water - hydrogen
bonding (stronger)
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Clausius-Clapeyron Equation
Vapor Pressure vs Temperature
Pressure 
ln Pvap   
H vap  1 
   C
R
T 
liquid
solid
gas
Temperature 
Clausius-Clapeyron Equation
Vapor Pressure vs Temperature
Plot of ln(P) vs 1/T yields
straight line:
• Slope = −ΔHvap/R
• Intercept = constant
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Sample Exercise 11.2:
Calculating Vapor Pressure Using Hvap
The compound with the common name isooctane has the structure
shown below. Its normal boiling point is 99 oC, and Hvap = 35.2 kJ/mol.
What is the vapor pressure at 25 oC?
Practice Exercise, p. 473
Pentane (C5H12) gas is used to blow the bubbles in molten polystyrene that turn it into
Styrofoam, which is used in coffee cups and other products that have good thermal
insulation properties. The normal boiling point of pentane is 36 oC; its vapor pressure
at 25 oC is 505 torr. What is the enthalpy of vaporization of pentane?
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