• Vapor Pressure:
o Vapor Pressure:
Pressure exerted by a gas in equilibrium with its liquid.
Rates of evaporation and condensation are
PV = nRT
P = nRT / V
Generally, vapor pressure of solution is lower than vapor pressure
of pure solvent.
Number of particles affect vapor pressure (type of particles does
• Factor Affecting Vapor Pressure
Higher temperature = higher kinetic energy; more molecules with
sufficient kinetic energy to overcome attractive forces in liquid
o Intermolecular Forces:
Must be overcome for molecules to leave the liquid and enter gas
Review of the types and relative strengths
• Vapor Pressure of Solutions:
o Raoult’s Law:
vapor pressure of solution is proportional to mole fraction of
solvent (w. nonvolatile solute).
o Vapor Pressure Lowering: one of the colligative properties of solutions
o Ideal Solutions: solutions that obey Raoult’s Law
• Vapor Pressure vs Temperature:
o Normal boiling point: the temperature at which the vapor pressure of a
liquid equals 1 atm (760 mmHg).
o ClausiusClapeyron Equation:
ΔH vap 1
ln Pvap = − + C
Plot of ln(P) vs 1/T yields straight line:
• Slope = −ΔHvap
• Intercept = constant
o For a given liquid at two different temperatures:
ln vap,1 = vap 1 − 1
P R T T
vap,2 2 1 • Fractional Distillation:
o Process to separate volatile components of a mixture.
o Vapor phase enriched in more volatile component:
As vapor condenses, evaporates again, achieve greater enrichment.
Ideally, separate and collect pure components.
• Solutions of Volatile Components:
o For mixtures containing more than one volatile component:
Partial pressure of each volatile component contributes to total
vapor pressure of solution.
P totalX 1 °1+ X P2° 2 X P 3 …3
Where X = mole fra