GCSU – Chem 1212L Experiment
#4
Chemistry with Calculators 4 - 1
Using Freezing-Point Depression
to Find Molecular Weight
When a solute is dissolved in a solvent, the freezing temperature is lowered in proportion to the
number of moles of solute added. This property, known as freezing-point depression, is a
colligative property; that is, it depends on the ratio of solute and solvent particles, not on the
nature of the substance itself. The equation that shows this relationship is:
Δt = Kf • m
where Δt is the freezing point depression, Kf is the freezing point depression constant for a
particular solvent (3.9°C-kg/mol for lauric acid in this experiment1), and m is the molality of the
solution (in mol solute/kg solvent).
In this experiment, you will first find the freezing temperature of the pure solvent, lauric acid,
CH3(CH2)10COOH. You will then add a known mass of benzoic acid solute, C6H5COOH, to a
known mass of lauric acid, and determine the lowering of the freezing temperature of the
solution. In an earlier experiment, you observed the effect on the cooling behavior at the freezing
point when a solute was added to a pure substance. By measuring the freezing point depression,
Δt, and the mass of benzoic acid, you can use the formula above to find the molecular weight of
the benzoic acid solute, in g/mole.
Figure 1
MATERIALS
TI-83 Plus or TI-84 Plus graphing calculator
400 mL beaker
EasyData application
18 X 150 mL test tube
Temperature Probe and data-collection interface
lauric acid
or EasyTemp
benzoic acid
ring stand
thermometer
utility clamp
water
1 “The Computer-Based Laboratory,” Journal of Chemical Education: Software, 1988, Vol. 1A, No. 2, p. 73.
