Colligative Properties: Freezing-Point Depression and Molar Mass
, Doug Gately
: To become familiar with colligative properties and to use them to determine the
molar mass of a substance.
: 600-mL beaker, thermometer, large test tube, 250-mL wide-mouth glass
bottle, paper towels, wire gauze, clamp, standard laboratory balance, analytical balance,
Bunsen burner, rubber hose, wire stirrer, weighing paper, ring stand, iron ring, two-hole
rubber stopper with slit
: sulfur, naphthalene
: Solutions are homogeneous mixtures that contain two or more substances.
The major component is called the
, and the minor component is called the
Since the solution is primarily composed of solvent, physical properties of a solution
resemble those of the solvent. Some of these physical properties, called
, are independent of the nature of the solute and depend only upon the solute
concentration, measured in
, or moles of solute per kilogram of solvent.
The colligative properties include vapor pressure lowering, boiling point
elevation, freezing point depression, and osmotic pressure. The vapor pressure is the
escaping tendency of solvent molecules. When the vapor pressure of a solvent is equal to
atmospheric pressure, the solvent boils. At this temperature, the gaseous and liquid states
of the solvent are in dynamic equilibrium, and the rate of molecules going from the liquid
to the gaseous state is equal to the rate of molecules going from the gaseous state to the
The phase diagram below illustrates the effect of adding a solute to a pure