The Foundations of Chemistry

LOWERING OF VAPOR PRESSURE AND RAOULT’S LAW

Many experiments have shown that a solution containing a nonvolatileliquid or a solid as
a solute always has a lower vapor pressure than the pure solvent (Figure 14-8). The vapor
pressure of a liquid depends on the ease with which the molecules are able to escape from
the surface of the liquid. When a solute is dissolved in a liquid, some of the total volume
of the solution is occupied by solute molecules, and so there are fewer solvent molecules
per unit areaat the surface. As a result, solvent molecules vaporize at a slower rate than if
no solute were present. The increase in disorder that accompanies evaporation is also a
significant factor. Because a solution is already more disordered (“mixed up”) than a pure
solvent, the evaporation of the pure solvent involves a larger increase in disorder, and is
thus more favorable. Hence, the pure solvent exhibits a higher vapor pressure than does
the solution. The lowering of the vapor pressure of the solution is a colligative property.
It is a function of the number, and not the kind, of solute particles in solution. We empha-
size that solutions of gases or low-boiling (volatile) liquid solutes can have highertotal
vapor pressures than the pure solvents, so this discussion does not apply to them.
The lowering of the vapor pressure of a solvent due to the presence of nonvolatile,
nonionizingsolutes is summarized by Raoult’s Law.

14-9

A vaporis a gas formed by the boiling

or evaporation of a liquid or

sublimation of a solid. The vapor

pressureof a liquid is the pressure

(partial pressure) exerted by a vapor in

equilibrium with its liquid (see Section

13-7).

14- 9Lowering of Vapor Pressure and Raoult’s Law 557

Figure 14-8 Lowering of vapor pressure. If no air is present in the apparatus, the pressure
above each liquid is due to water vapor. This pressure is less over the solution of sugar and
water.

Water Sugar

solution

Manometer