Vapor Pressure of Solutions

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Slide 1

Vapor Pressure

Francois Marie Raoult

William Henry

Slide 2

Henry’s Law

The concentration of a dissolved gas in a solution is directly proportional to the pressure of the gas above the solution

Applies most accurately for dilute solutions of gases that do not dissociate or react with the solvent

Yes  CO2, N2, O2

No  HCl, HI

Slide 3

Raoult’s Law

The presence of a nonvolatile solute lowers the vapor pressure of the solvent.

Psolution = Observed Vapor pressure of

the solution

P0solvent = Vapor pressure of the pure solvent

solvent = Mole fraction of the solvent

Slide 4

Liquid-liquid solutions in which both components are volatile

Modified Raoult's Law:

P0 is the vapor pressure of the pure solvent

PA and PB are the partial pressures

Slide 5

Raoult’s Law – Ideal Solution

A solution that obeys Raoult’s Law is called an ideal solution

Slide 6

Negative Deviations from Raoult’s Law

Strong solute-solvent interaction results in a vapor pressure lower than predicted

Exothermic mixing = Negative deviation

Slide 7

Positive Deviations from Raoult’s Law

Weak solute-solvent interaction results in a vapor pressure higher than predicted

Endothermic mixing = Positive deviation