Molar Mass by Freezing Point Depression
Adapted from Vonderbrink, Laboratory Experiments for AP Chemistry
A procedure that allows the determination of the molar mass of a substance is very useful to chemists. The
molar mass is an important value that must be known in order identify an unknown substance or to characterize
a newly prepared compound.
There are a number of ways of determining the molar mass of a substance. One of the simplest involves
finding the change in the freezing point of a solvent when an unknown substance is dissolved in it. It has been
found that the change in freezing point is directly proportional to the molality of the solution. This change in
freezing point is one of several "colligative" properties of solutions, properties that depend only on the number
of dissolved particles in solution, and not on the type of particle. Other colligative properties include change in
boiling point, vapor pressure and osmotic pressure. Measurements of these properties also can be used to find
molecular mass of solute.
The relation to change in freezing point is:
∆Tfp = kfpm
where Tfp is the change in freezing point, kfp is the freezing point depression constant for the solvent, and m is
the molality of the solution. The value of kfp must be determined for each solvent.
The solvent that will be used in this experiment is a nonpolar solvent with the
common name butylated hydroxytoluene. This compound is abbreviated BHT and
is frequently used as an antioxidant in foods. The IUPAC name for the compound
is 2,6-di-tert-butyl-4-methylphenol. Its structural formula is:
The figure below shows a cooling curve for a pure solvent and for a solution.
Notice that supercooling may occur in both the solvent and the solution. If it does, as the crystals begin to form
the temperature will rise slightly and then remain constant as the pure solvent freezes or will slowly fall as the
solution freezes.
Experiment Overview
The purpose of this experiment is to determine the molar mass of an unknown substance by measuring the
freezing point depression of a solution of the unknown substance and BHT.
Part 1. The freezing point of BHT is first determined by analyzing a cooling curve for BHT. Even though the
freezing point of BHT is known, it is necessary to determine it with the thermometer that will be used in the
experiment. Even if the thermometer reading is slightly off, the change in temperature should be accurate. It is
important that the same thermometer is used to determine both the freezing point temperature of the solvent and
that of the solution.
Part 2. A known amount of stearic acid (285 g/mol) is then added to a measure quantity of BHT. The freezing
point depression (∆Tfp) of this solution is found and the freezing point depression constant (kfp) is calculated.
Part 3. The unknown is added to BHT, the freezing point depression of this solution is measured, and the molar
mass of the unknown is then determined.
To save time, Parts 1 and 2 will be done as a “dry lab”; i.e. you will be given the data. You will need to analyze
the data to determine kfp. You will then use a microscale technique to redo Part 2 (melting point of BHT) and to
do Part 3 (melting point of a mixture of BHT plus an unknown). From the freezing point depression of the
mixture, you will calculate the molar mass of the unknown.
Part 2-3 Procedure
1. Pulverize a small amount (about 0.5 g) of BHT with a mortar and pestle.
2. Pack the BHT in a capillary tube to a depth of about 1 cm. Fasten the capillary tube to a digital
thermometer. The BHT should be level with the bulb of the thermometer.
3. Immerse the thermometer+capillary tube assembly in a Thiele melting point
tube filled with water and heat. Heating can be rapid in the beginning, but as
the temperature approaches the melting point,  67°C, heat very slowly in
order to get an accurate value.
4. Record the temperature at which the BHT melts. If repeating the melting
procedure, use a new sample and capillary tube.
5. Use an analytical balance to accurately measure about 0.5 g ( 1 mg) BHT
into a small beaker. Accurately measure about 0.1 g ( 1 mg) of unknown
into the beaker. Heat the beaker gently over a small flame. When the
substance melts, mix it well with a stirring rod until it is homogeneous.
Allow the mixture to cool and solidify.
6. After it solidifies, pulverize a small amount of the mixture and determine the
solution melting point.
Cooling Cuves
Data for BHT kfp Determination
Mass of BHT, g
7.990 g
Mass of stearic acid, g
1.656 g
Time, s
0
20
40
60
80
100
120
140
160
180
200
220
240
260
300
360
Temperature, °C
BHT +
BHT
stearic acid
85.0
76.8
80.0
74.7
75.8
74.5
72.0
72.2
69.0
69.8
68.8
67.8
69.0
65.9
68.8
64.3
68.6
62.9
68.4
61.6
68.2
60.4
61.1
61.5
61.6
61.2
60.5