Molar Mass by Freezing-Point Depression
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
t = Kf  m  i
wheret 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 experiment), m is the molality of the
solution (in mol solute/kg solvent), and i is the van’t Hoff factor.
The pure solvent is a nonpolar compound called 2,6-ditertbutyl-4methylphenol. A more common name for the compound is butylated
hydroxytoluene (BHT). This compound is frequently used in food
additives. Its structural formula is shown at the right.
MATERIALS LabQuest
ring stand
Temperature Probe
two 18  150 mm test tubes
400 mL beaker
2,6-di-tert-butyl-4-methylphenol, BHT
Cetyl alcohol, CH3(CH2)14CH2OH
hot water bath
utility clamp
paper towel
wigh paper
Pre‐Lab Question 1. The following data were obtained in an experiment designed to find the molar mass of a
solute by freezing point depression
Solvent para-dichlorobenzene
Kf for solvent: 7.1 ºC/m
Freezing point of pure solvent: 53.02 ºC
Mass of para-dichlorobenzene 24.80 g
Mass of unknown substance: 2.04g
Freezing point of solution: 50.78 ºC
a. Determine the freezing point depression
b. Calculate the molar mass of the unknown substance.
PROCEDURE 1. Obtain and wear goggles and aprons.
2. Connect the Temperature Probe to LabQuest and choose New from the File menu. If you
have an older sensor that does not auto-ID, manually set up the sensor.
3. On the Meter screen, tap Rate. Change the data-collection rate to 1 sample/second and the
data-collection length to 600 seconds.
Part I: Freezing Temperature of Pure BHT 1. Add about 250 mL of tap water to a 400-mL beaker. Place the beaker on the hotplate and
turn it to the highest setting.
2. Accurately measure about 8 grams of BHT into the test tube using a sensitive balance.
Record the exact mass.
3. Clamp the test tube in the water bath and insert the temperature probe into the BHT. About
30 seconds are required for the probe to warm up to the temperature of its surroundings and
give correct temperature readings. Heat the water bath to about 90ºC.
4. Allow the BHT in the test tube to melt.
5. Begin data collection. Raise the test tube out of the hot water bath and observe the
temperature as the melted BHT cools. It is important to continuously stir the BHT to
maintain even cooling. Stirring will also help prevent supercooling.
6. Make a note of the temperature where crystals begin to form. Stir until the BHT solidifies.
7. The freezing temperature can be determined by finding the mean temperature in the portion
of the graph with nearly constant temperature.
a. Select the data point at the beginning of the flat portion of the graph and drag across the
flat portion to select the region.
b. Choose Statistics from the Analyze menu.
c. Record the mean (average) temperature as the freezing temperature of pure BHT.
8.
Store the data from the first run by tapping the File Cabinet icon.
Part II Determine the Freezing Point of a Solution of BHT and cetyl alcohol
1. Using an analytical balance accurately measure about 1 g of cetyl alcohol onto a piece of
waxed weighing paper and record its exact mass.
2. Place the ceytl alcohol into the test tube containing BHT.
3. Clamp the test tube into the water bath and insert the temperature probe. Heat the mixture in
the hot water bath until the substances are all melted. Stir well to ensure the solution is
homogeneous.
4. Repeat steps 5-8 from part I.
5. When data collection is complete, use a hot water bath to melt the BHT/cetyl alcohol enough
to safely remove the Temperature Probe. Carefully wipe any excess liquid from the probe
with a paper towel or tissue.
Part III Determine the Freezing Point of a Solution of BHT and unknown
1. Repeat experimental steps using fresh BHT, a clean test tube, temperature probe and about 1
g of the unknown compound in place of 1 g of cetyl alcohol.
2.
When you have completed Step 9, examine the data pairs on the displayed graph. The
freezing point of the cetyl alcohol-BHT solution can be determined by finding the
temperature at which the mixture initially started to freeze. Unlike pure BHT, the mixture
results in a gradual linear decrease in temperature during freezing. As you move the examine
line, the temperature and time values are displayed to the right of the graph. Locate the initial
freezing temperature of the solution, as shown above. Record the freezing temperature in
your data table.
3. Tap Run 2 and select All Runs to view both trials on the same graph.
4. Print the graph directly from LabQuest, if possible. Plot your data to show all trials on the
same graph and print a copy of the graph.
Analysis: 1. From the graph, determine the freezing points of the solvent and of each of the solutions, and
the values of ∆T, between the pure BHT and the mixture of BHT and cetyl alcohol, and
calculate the molality of the BHT- cetyl alcohol mixture
2. Calculate the experimental Kf, the freezing point depression constant, for BHT from the
molality and the change in the freezing point temperature of the cetyl alcohol solution.
3. Calculate the molar mass of the unknown solute using the freezing point depression constant,
the mass of the unknown solute, and the mass of the BHT.
4. Calculate the percent error of the unknown, contact the instructor for the actual molar mass.
Error Analysis 1.The following errors occurred when the above experiment was carried out. How would each
affect the calculated molecular mass of the solute (too high, too low, no effect)? Explain.
(this can take the
a. The thermometer used actually read 1.4'C too high.
b. Some of the solvent was spilled before the solute was added.
c. Some of the solute was spilled after it was weighed and before it was added to the
solvent.
d. Some of the solution was spilled after the solute and solvent were mixed but before
the freezing point was determined.
To be included in the conclusion 1.What was the least precise measurement in the experiment? How does this limit the significant
digits?
2.Did the solutions show any evidence of supercooling?
3.Why is it advantageous to choose a solvent that has a large value for Kf?