Determination of 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
ΔTf = Kf × m× i where ΔTf 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), i is the van’t Hoff factor, and m is the molality of the solution (in mol solute/kg solvent). Since lauric acid is not ionic, its van’t Hoff factor is essentially equal to 1.
OBJECTIVES
In this experiment, you will
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Determine the freezing temperature of the pure solvent, lauric acid.
Determine the freezing temperature of a mixture of lauric acid and benzoic acid.
Calculate the freezing point depression of the mixture.
Calculate the molecular weight of benzoic acid.
Figure 1
MATERIALS
Data Collection Mechanism
Temperature Probe ring stand
400 mL beaker
Tissue or paper towels
lauric acid, CH3(CH2)10COOH lauric acid-benzoic acid mixture hot water bath utility clamp two 18 × 150 mm test tubes (if pre-made samples are not provided by your teacher)
1
“The Computer-Based Laboratory”, Journal of Chemical Education: Software, 1988, Vol.1A, No. 2, p. 73.
Adapted from Advanced Chemistry with Vernier & Laboratory Experiments for Advanced Placement Chemistry by Sally Ann Vonderbrink, Ph. D.
The Determination of Molar Mass by Freezing-Point Depression
PROCEDURE
1. Obtain and wear goggles.
2. Set up the data collection system.
a. Connect a Temperature Probe to the interface.
b. Start the data collection program.
c. Set up the time graph for 10 seconds per sample and 60 samples.
Part I: Determine the Freezing Temperature of Pure Lauric Acid
3. Add about 300 mL of tap water with a temperature of 20-25°C to a 400 mL beaker. Place the beaker on the base of the ring stand.
4. Use a utility clamp to obtain a test tube containing hot, melted lauric acid from your instructor. Fasten the utility clamp at the top of the test tube. CAUTION: Be careful not to spill the hot lauric acid on yourself and do not touch the bottom of the test tube.
5. Insert the Temperature Probe into the hot lauric acid. Fasten the utility clamp to the ring stand so the test tube is above the water bath.
6. Begin data collection. Lower the test tube into the water bath. Make sure the water level outside the test tube is higher than the lauric acid level in the test tube, as shown in Figure 1.
7. With a very slight up-and-down motion of the Temperature Probe, continuously stir the lauric acid for the ten-minute duration of the experiment. Do not allow the temperature probe to touch the bottom of the test tube.
8. When data collection is complete, use a hot water bath to melt the lauric acid enough to safely remove the Temperature Probe. Carefully wipe any excess lauric acid liquid from the probe with a paper towel or tissue. 9. 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 in the flat region of the graph.
b. Find the mean temperature for the selected data. Record this value.
c. Store the data, so that they can be used later.
Adapted from Advanced Chemistry with Vernier & Laboratory Experiments for Advanced Placement Chemistry by Sally Ann Vonderbrink, Ph. D.
The Determination of Molar Mass by Freezing-Point Depression
Part II: Determine the Freezing Point of a Solution of Benzoic Acid and Lauric Acid
10. Obtain a test tube containing a melted solution with ~1 g of benzoic acid dissolved in ~8 g of lauric acid.
Record the precise masses of benzoic acid and lauric acid as indicated on the label of the test tube.
Freezing Point
