Lab 10: Determining Molar Mass by Freezing Point
Depression
Background Information:
Chemistry is the study of physical properties of substances. The colligative properties of dissolving
lauric acid in benzoic acid will be examined through the exploration of the impact on the solutions freezing
point. Colligative properties are those properties that depend only on the amount of substance that is present,
not the identity of the substance. Examples of these properties include vapor pressure lowering, boiling point
elevation, freezing point depression, and osmotic pressure. In this lab, you will be exploring how the freezing
point of a substance can be altered as more or less substance is dissolved.
This experiment will use lauric acid as the solvent and benzoic acid as the solute. As you add more
benzoic acid to lauric acid, the freezing point is observed to decrease. This is because as you add more solute, it
interferes with solvent – solvent intermolecular forces that occur as a substance begins to cool. This causes the
freezing point to be lowered due to this increased crowding at the molecular level. The amount of solute will be
varied in order to calculate numerous parameters. The observed process can be explained through the following
equation:
Tf  i K f  Molality
where T is the change in freezing point, Kf is the freezing point constant (depends on the solvent used), and m is
the concentration of the solute expressed a molality. Molality is calculated by the equation:
moles of solute
Molality 

kg of solvent
The variable, i, is the van’t Hoff factor and it describes the number of particles a substance would break into as
it dissolve. For example, ionic substances may have a van’t Hoff factor greater than 1. Sodium chloride, NaCl,
would have a van’t Hoff factor of i=2. Magnesium chloride, MgCl2, would have a factor of i=3. Molecular
The substance that are being used in this experiment are molecular, so i=1.
substances will have a factor of 1.
Pre – Lab
1. As you add more solute to a solution, why does the freezing point decrease.
2. You have a solution that contains 14 g of NaCl dissolved in 50 kg of water. Calculate the freezing point
depression for this solution. Note Kf for water is ___________.
Materials
 Lauric acid, C12H24O2
 Benzoic acid
 Tap Water
 Stop – Watch or Timer
 Alcohol burner
 Thermometer




Ring Stand
Test Tube
Test Tube Clamp
Large Beaker (600 mL or 800
mL)
Procedure
Part A: Determination of the melting point of pure lauric acid
1. Set up ring stand with clamp under the alcohol burner
2. Use a small piece of paper that has been creased in the middle, weigh out about 10 g of solid lauric acid.
Record the actual mass of the lauric acid used. Place the lauric acid into the test tube.
3. Fill the large beaker about ¾ full with tap water
4. Place beaker with water on hot plate and heat the water
5. Place test tube in beaker and heat until the lauric acid is completely melted (be sure to stir the laurc acid
carefully with the thermometer)
6. Once the lauric acid is completely melted, carefully remove the lauric acid from the hot water and allow it to
cool. Keep the beaker of the warm water for later trials.
7. Record the temperature every 30 seconds until the temperature reaches approximately 35 oC
Part B: Determination of the freezing point of lauric acid/benzoic acid mixture
8. Use a small piece of paper that has been creased in the middle, weigh out about 1.0 g of benzoic acid and
place into the test tube with lauric acid. Record the actual mass of the benzoic acid used.
9. Using the hot water from the trial with the pure lauric acid, place test tube in the beaker and heat until the
lauric acid/benzoic acid mixture is completely melted (Be sure to stir the lauric acid carefully with the
thermometer)
10. Once the mixture is completely melted, carefully remove it from the hot water and allow it to cool.
11. Record the temperature every 30 seconds until the temperature reaches approximately 25 oC
12. Repeat steps 8 through 11 two more times increasing the amount benzoic acid by about 1.0 g each time.
Record the total mass of benzoic acid used with each trial.
Table 1: Observations
Time
Trial 1 (______ g
lauric acid)
Trial 2 (_____ g
lauric acid + _____g
benzoic acid)
Trial 3
(_____ g lauric acid +
_______ g benzoic acid)
Trial 4
(_____ g lauric acid +
_______ g benzoic acid)
0:30
1:00
1:30
2:00
2:30
3:00
3:30
4:00
4:30
5:00
5:30
6:00
6:30
7:00
7:30
Calculations:
1. Graph the temperature data with temperature on the y – axis and time on the x – axis. If done carefully, all
three sets of data can be plotted on the same graph in different colors. Label all axis and scales. Provide a
legend and a title.
2. Determine the freezing point for each trial from the graph. Label them on the graph.
3. Determine the ∆Tf for each trial.
4. The Kf for lauric acid is 3.90 oC kg/mol. Use this information, along with the data from the lab to calculate
the molality of each solution.
5. Using the molality equation and the data from the lab, calculate the molar mass of benzoic acid from each
trail. Average to get a final answer.
Discussion Questions
1. The theoretical molecular formula for benzoic acid is C7H6O2. Calculate the molar mass of this substance. Is
there a difference between the theoretical molar mass and the molar mass that you calculated? Why do you
think there were differences?
2. Assume you massed out more benzoic acid than you initially recorded in your data table. How would this
impact your final calculated molar mass?
3. How can this experiment be altered in the future so as to obtain more accurate results? Give at least 3 areas
that require changes.