Colligative Properties of Solutions:
Freezing Point Depression and Molar Mass Determination
When a solute is dissolved in a solvent, the properties of the solvent are changed
by the presence of the solute. The magnitude of the change generally is proportional to
the amount of solute added. Some properties of the solvent are changed only by the
number of solute particles present, without regard to the particular nature of the solute.
Such properties are called Colligative properties of the solution. Colligative properties
include changes in vapor pressure, boiling point, freezing point, and osmotic pressure.
For example, if a nonvolatile, nonionizing solute is added to a volatile solvent,
(such as water), the amount of solvent that can escape from the surface of the liquid at a
given temperature is lowered, relative to the case where only the pure solvent is present.
The vapor pressure above such a solution will be lower than the vapor pressure above a
sample of the pure solvent under the same conditions. Molecules of nonvolatile solute
physically block the surface of the solvent, thereby preventing as many molecules from
evaporating at a given temperature. As shown in the figure, if the vapor pressure of the
solution is lowered, there is an increase in the boiling point of the solution as well as a
decrease in the freezing point.
In this experiment, you will determine the freezing points of a pure solvent (lauric
acid), a solution of a known solute (camphor, C6H4Cl2) dissolved in lauric acid, and an
unknown solution of sulfur in lauric acid. The sulfur is considered to be an unknown
solute because you will be determining the molar mass of sulfur (and the formula of
elemental sulfur). It may turn out to be rather surprising.
The decrease in freezing point (Tf) when a nonvolatile, nonionizing solute is
dissolved in a solvent is proportional to the molal concentration (m) of solute present in
the solvent:
Tf = Kfm
Kf is a constant for a given solvent (called the molal freezing point depression
constant) and represents by how many degrees the freezing point will change when 1.00
mol of solute is dissolved per kilogram of solvent. For example, Kf for water is
1.86oCkg/mol, whereas Kf for the solvent benzene is 5.12oCkg/mol.
The molal concentration of a solution represents how many moles of solute are
dissolved per kilogram of the solvent. For example, if 0.50 mol of the sugar sucrose were
dissolved in 100 g of water, this would be equivalent to 5.0 mol of sugar per kilogram of
solvent, and the solution would be 5.0 molal. The molality of a solution is defined as
molality, m = moles solute/kilograms solvent
The measurement of freezing point lowering is routinely used for the determination of the
molar masses of unknown solutes. If both of the equations above are combined, it can be
derived that the molar mass of a solute is related to the freezing point lowering
experienced by the solution and to the composition of the solution.
In the preceding discussion, we considered the effect of a nonionizing solute on
the freezing point of a solution. If the solute doe indeed ionize in the solvent, the effect
on the freezing point will be larger. The depression of the freezing point of a solvent is
related to the number of particles of solute present in the solvent. If the solute ionizes as
it dissolves, the total number of moles of all particles present in the solvent will be larger
than the formal concentration indicates. For example, a 0.1 m solution of NaCl is
effectively 0.1m in both Na+ and Cl- ions.
Safety Precautions:
 Protective eyewear approved by your institution must be worn at all times while
you are in the laboratory.
 Lauric acid and camphor are toxic, and their vapors are harmful. Do not handle
these substances. Work in a well ventilated area. Wear disposable gloves. These
substances are also combustible and should be kept away from open flame.
 Dispose of the lauric acid solutions while they are warm!
Procedure
A. Determination of the Freezing Point of Lauric acid












Set up a large beaker about ¾ full of water and heat the water to boiling.
Insert your thermometer through a one (or two) hole stopper so that the
temperature can still be read from 60oC to 90oC. Also use this to stir the
solution.
Weigh a clean, dry 8-inch test tube to the nearest 0.01 gram.
Add approximately 10 g of lauric acid to the test tube, and reweigh to the
nearest 0.01 g.
Use a clamp to set the test tube vertically in the boiling water bath. Allow
the lauric acid to melt completely.
When the lauric acid has melted completely, insert the rubber stopper
containing the thermometer into the test tube. Make certain that the bulb
of the thermometer is immersed in the molten lauric acid.
Continue to heat the test tube to remelt any lauric acid that might have
crystallized on the thermometer. Move the thermometer up and down to
make certain the temperature is uniform throughout the lauric acid sample.
When the lauric acid has re-melted completely (generally the temperature
will be above 95oC), remove the beaker of hot water from under the test
tube using tongs or a towel to protect your hands.
Ensure that the thermometer is immersed in the center of the molten lauric
acid and does not touch the walls of the test tube. When the temperature of
the molten lauric acid has dropped to 90oC, begin recording the
temperature of the lauric acid every 30 seconds. Record the temperature
sot the nearest 0.1oC.
Continue to take the temperature of the lauric acid until the temperature
has dropped to around 40-45oC. The lauric acid will freezing in this
temperature range.
Do not discard the lauric acid!!!!
Rinse the thermometer to remove any excess lauric acid with hot water.
B. Determination of Kf for Lauric acid







Reweigh the test tube containing the lauric acid to the nearest 0.01g.
Some of the lauric acid may have been lost on the thermometer/stirrer or
may have vaporized.
Add approximately 1g of camphor crystals to the test tube containing the
lauric acid, and reweigh the test tube to the nearest 0.01g.
Melt the lauric acid/camphor mixture in boiling water.
Make certain that all acetone has evaporated from the thermometer/stirrer
apparatus (any remaining in acetone would also lower the freezing point
of lauric acid).
Insert the thermometer and stirrer into the test tube containing the molten
lauric acid/camphor mixture. Stir the mixture for several minutes while it
is in the boiling water bath to make certain that the solution is completely
homogeneous.
Following the same procedure as in Part A, determine the cooling curve
and the freezing point of the lauric acid/camphor solution.
Re-melt and discard the lauric acid/camphor solution in the waste
container indicated by your instructor.
C. Determination of the Freezing Point of a Sulfur Solution






Clean and dry an 8-inch test tube, and weigh it to the nearest 0.01 g.
Add approximately 10 g of lauric acid to the test tube, and reweigh the test
tube and contents to the nearest 0.01g.
Add approximately 0.5 g of powdered sulfur to the lauric acid in the test
tube, and reweigh again to the nearest 0.01 g.
Make certain that all acetone has evaporated from the thermometer/stirrer
apparatus.
Melt the mixture in the boiling water bath, and insert the thermometer.
Stir thoroughly to make certain that all the sulfur is dissolved. Again
follow the same procedure as earlier to determine the cooling curve and
freezing point of lauric acid/sulfur.
Remelt and discard the solution as instructed by your instructor. Wash the
thermometer and stirrer with water and soap.
Analysis and Conclusions
On graph paper or excel, graph a cooling curve for pure lauric acid; graph the
temperature of the lauric acid as it cools versus the time in minutes. You will notice that
the lauric acid remains at the same temperature for several minutes. This results in a
horizontal, flat region on the cooling curve. This temperature represents the freezing
point of pure lauric acid.
Create cooling curves for both the lauric acid/camphor and the lauric acid/sulfur mixtures
as well. Attach all three graphs to your lab report.
Use the freezing point depression and the solution composition of lauric acid/camphor to
calculate the molal freezing point depression constant, Kf, for lauric acid. The molecular
formula of camphor is C6H4Cl2.
From the graph of freezing point depression of the sulfur/lauric acid solution and Kf for
lauric acid as determined in Part B, calculate the molar mass of sulfur. Determine the
number of sulfur atoms that must be present in a molecule of elemental sulfur to give rise
to this molar mass.
Questions to include in your conclusions:
1. A phenomenon called supercooling is frequently encountered in this experiment.
In supercooling, a solution momentarily drops below its freezing point, and then
warms up again, before solidification. What event is likely to give rise to
supercooling?
2. Look up the freezing point constant, Kf, for lauric acid in a handbook. Calculate
the percent error between your determination of Kf and the handbook value.
What factors might have led to your obtaining a different value?