Experiment 1

Freezing Point Depression of Electrolytes

Colligative properties are properties of solutions that depend on the concentrations of the

samples and, to a first approximation, do not depend on the chemical nature of the samples. A

colligative property is the difference between a property of a solvent in a solution and the same

property of the pure solvent: vapor pressure lowering, boiling point elevation, freezing point

depression, and osmotic pressure. We are grateful for the freezing point depression of aqueous

solutions of ethylene glycol or propylene glycol in the winter and are continually grateful to

osmotic pressure for transport of water across membranes. Colligative properties have been used

to determine the molecular weights of non-electrolytes.

Colligative properties can be described reasonably well by a simple equation for solutions

of non-electrolytes. The “abnormal” colligative properties of electrolyte solutions supported the

Arrhenius theory of ionization. Deviations from ideal behavior for electrolyte solutions led to the

determination of activity coefficients and the development of the theory of interionic attractions.

The equation for the freezing point depression of a solution of a non-electrolyte as a

function of molality is a very simple one (as you may remember from GEN CHEM):

Δ TF = K F m

(1)

The constant KF, the freezing point depression constant, is a property only of the solvent, as

given by the following equation, whose derivation is available in many physical chemistry texts1:

( )

o 2

MW(Solvent)R TF

KF =

1000 Δ H F

(2)

In equation (2), R is the gas constant in J/K*mol, TFo is the freezing point of the solvent (K), ΔHF

is the heat of fusion of the solvent in J/mol, and the factor of 1000 is needed to convert from g to

kg of water for molality. For water, KF = 1.860 o/molal from the properties of pure water and

from experimental data on the freezing point depressions of dilute...