Continuation of CHAPTER 11
CHEM 1212
Vapor Pressure
• Vapor pressure is the pressure exerted
when the liquid and vapor are in
dynamic equilibrium.
• Dynamic Equilibrium - the point when as
many molecules escape the surface as strike
the surface.
Prentice Hall © 2003
Chapter 11
What you have learned so far…….
• Some compounds are volatile (evaporates
easily) because… (a) the molecules are held
together by weak intermolecular forces 
therefore it doesn’t take much E to break the
bonds  and so they evaporate easily.
• Compounds that are volatile have lower
boiling points.
Chapter 11
Clausius-Clapeyron Equation
• ln P =
- DHvap
+
C
RT
Where:
T = absolute temperature
R = gas constant (8.314 J/K-mol)
DH = heat of vaporization
P = vapor pressure
C = constant
IDEAL GAS CONSTANT
• R = 0.08206 liter . atm/K . Mole
• OR
• R = 8.314 J/K . Mole
• Conversion 101.325 J = 1 L.atm
Vapor Pressure and Boiling Pt.
• Sample Problem
• If the normal boiling point of water is 100. oC,
what will be its boiling point at 735 torr? The
heat of vaporization of water is 40.67
kJ/mole.
Clausius-Clapeyron Equation
• To determine the effect of changing
temperature or vapor pressure, use the
following equation:
• ln [P1/P2] =
- DHvap [1/T1 - 1/T2]
R
Clausius-Clapeyron Equation
So your very first equation in Chem 1212
is:
• ln [P1/P2] =
DHvap [1/T2 - 1/T1]
R
Problem
CCl4 has a vapor pressure of 213
o
torr at 40 C and 836 torr at 80
oC. What is the heat of
vaporization of CCl4?
Extra Problem
• The melting point of potassium is 63.2 oC.
Molten potassium has a vapor pressure of
10.0 torr at 443 oC and a vapor pressure of
400.0 torr at 708 oC.
• A.) Calculate the heat of vaporization of
liquid potassium.
• B.) Calculate the normal boiling point of
potassium.
• C.) Calculate the vapor pressure of liquid
potassium at 100 oC.