CHEM1212L
Freezing point depression
Page 1 of 3
Some physical properties depend on the number of molecules or particles in the system rather than on their chemical nature. Such properties are called Colligative Properties .
There are four common colligative properties including vapor pressure lowering, boiling point elevation, freezing point depression , and osmosis . Chemists utilize these colligative properties in order to obtain both chemical and physical information about chemical systems. One of the common applications is the determination of molar mass of unknown chemicals. In this experiment, you will determine the freezing temperatures of the pure solvent, lauric acid; and a solution made by dissolving an unknown solute in lauric acid. The object of this experiment is to determine the molar mass of the unknown solute from its measured freezing point depression. The equation below shows the relationship between freezing point depression (

T f
) and molality (m) of the solute:

T f
= K f

m
Where K f
= molar freezing point depression constant of the solvent. For our lab today, the K f
for lauric acid is 3.9 C-kg/mol.
Procedure
1. Prepare a water bath using a 250mL beaker, a tripod, and a wire screen. Since the lauric acid is a solid at room temperature, you will use this water bath to melt it and let it cool back towards room temperature to determine its freezing point. Fill the beaker about 2/3 full with tap water and keep the water bath temperature higher than
60 o C by heating it with a Bunsen burner.
2. To conduct the experiment, place approximately 1g of lauric acid into a test tube.
Record the mass of lauric acid used to +/- 0.001g.
3. Place the test tube in a water bath to melt lauric acid.

CHEM1212L
Freezing point depression
Page 2 of 3
4. When there is no solid lauric acid observed on the bottom of the test tube, place a thermometer into the hot lauric acid and record the initial temperature of the lauric acid.
5. Take the test tube out of the water bath and record the temperature of the lauric acid every 30 second for 10 minutes or until one minute after it appears completely solidified.
6. Do NOT attempt to pull the thermometer out of the solid lauric acid. Use the hot water bath to melt the lauric acid, and then remove the thermometer.
7. Weigh approximately 0.1g of unknown solute. Record the mass of the unknown solute to +/- 0.001 g.
8. Carefully transfer all the unknown solute into the test tube containing the melted lauric acid.
9. Put the test tube back to water bath to melt the unknown solute. With a slight up and down motion of the thermometer, continuously, but gently, stir the solution while it is in the water bath and during the cooling.
10. Repeat steps 4, and 5.
11. After the solution is completely solidified, again melt the solid solution to remove the thermometer. Carefully wipe any excess solution from the thermometer with a paper towel. Pour the liquid solution into a designated waste container and clean the test tube with soapy water. Rinse 3X with small amounts of distilled water.
Data analysis:

Graph temperature (y-axis) as a function of time (x-axis) for both your runs.

Determine the freezing points for both pure lauric acid and the lauric acid/unknown chemical mixture from the freezing point graphs.

CHEM1212L
Freezing point depression
Page 3 of 3

Calculate the molality of the unknown chemical in the solution from your temperature data

Determine the experimental molar mass of the unknown chemical.

Your instructor will provide you with the known mass of the unknown chemical so you can determine the percent error in your determination.
Follow-up activities:
1. Which of the following factors is most likely responsible for your reported percent error: the mass measurements, the temperature measurements, the freezing point determinations from the graphs? Briefly explain your choice.
2. If your percent error is >10%, do your results indicate that the unknown chemical is partially or totally dissociated when it dissolves in lauric acid? Explain your reasoning.
3. If your error is less than 10% consider this question:
In water, the unknown chemical is considered a very weak electrolyte. Suppose the
Van't hoff factor for the unknown chemical dissolved in lauric acid is 1.1 instead of 1.0.
Reanalyze your data to determine the experimental mass of the unknown chemical you would have reported if you use i= 1.1. Based on your experimental results, are you confident that your unknown chemical did not dissociate in lauric acid? Explain your reasoning.