21
Colligative Properties
The colligative properties of solutions are those properties that depend upon the
number of dissolved molecules or ions, irrespective of their kind. They are the
lowering of the vapor pressure, the depression of the freezing point, the elevation of the boiling point, and the osmotic pressure. These properties may be
used in determining molecular weights of dissolved substances.
VAPOR PRESSURE
When a nonvolatile nonelectrolyte is dissolved in a liquid, the vapor pressure is
lowered. Raoult's law gives the mathematical relation
P = \P<,
(21-D
where P is the vapor pressure of the solution, P 0 is the vapor pressure of the
pure solvent, and x is the mole fraction of solvent in the solution. The lowering
of vapor pressure is not as widely used as some other colligative properties for
experimental determinations of molecular weights, because it is difficult to
make the measurements precisely.
PROBLEM:
The vapor pressure of liquid A (mole weight 120) is 70.0 torr at 25.0°C. What is the
vapor pressure of a solution containing 10.0 g C 6 H 4 C1 2 in 30.0 g of A?