Computer
Using Freezing-Point Depression
to Find Molecular Weight
4
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 = i × Kf × m
where ∆T is the freezing point depression; Kf is the freezing point depression constant for a
particular solvent; m is the molality of the solution (in mol solute/kg solvent), and i is the
so-called van’t Hoff factor, which is the number of moles of ions or molecules produced by one
mole of solute in the solution. For the molecular benzoic acid solute in this experiment, i = 1.
OBJECTIVES
In this experiment, you will
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•
•
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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
Vernier computer interface
Computer
Vernier Temperature Probe
Ring stand
400 mL beaker
Lauric acid, CH3(CH2)10COOH
Advanced Chemistry with Vernier
Benzoic acid, C6H6COOH
Hot water bath
Two 18 × 150 mm test tubes
Utility clamp
Paper towel or tissue
©Vernier Software & Technology
4-1
Computer 4
PROCEDURE
1. Obtain and wear goggles.
2. Connect a Temperature Probe to Channel 1 of the Vernier computer interface. Connect the
interface to the computer with the proper cable.
3. Start the Logger Pro program on your computer. Open the file “04 Freezing Point” from the
Advanced Chemistry with Vernier folder.
Part I Determine the Freezing Temperature of Pure Lauric Acid
4. Heat 350 mL of tap water in a 400 mL beaker on a hot plate. (Use a setting of 6 on the hot
plate.) While the water is heating, move on to the next step.
5. Weigh ~8 g. of lauric acid. Record the exact mass, and add it to the test tube provided.
6. Add about 300 mL of room temperature tap water to a 400 mL beaker. Place the beaker on
the base of the ring stand.
7. Secure the test tube containing lauric acid with a utility clamp attached to the rod on the hot
plate, similar to the diagram above. Lower the test tube into the hot water bath. Place the
temperature probe inside the test tube so that the tip of the probe rests at the bottom of the
tube. While the lauric acid is melting, go on to the next step.
8. When the lauric acid has reached approximately 60-65 °C, use the utility clamp to transfer
the test tube containing the hot, molten lauric acid from the hot plate to the ring stand. Fasten
to
the utility clamp to the ring stand so that the test tube is above the water bath. Click
begin the 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 inside the test tube, as shown in
Figure 1. CAUTION: Be careful not to spill the hot lauric acid on yourself, and do not touch
the bottom of the test tube. Perform this step quickly.
9. 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.
10. When the data collection is complete, use the 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.
11. The freezing temperature of pure lauric acid can be determined by finding the mean
temperature in the portion of the cooling curve for which the temperature is nearly constant.
a. Move the mouse pointer to the beginning of the graph’s flat part. Press the mouse button
and hold it down as you drag across the flat part of the curve, selecting only the points in
the plateau.
b. Click the Statistics button, .
c. The mean temperature value for the selected data is listed in the statistics box on the
graph. Record this value as the freezing temperature of pure lauric acid.
d. Click on the upper-left corner of the statistics box to remove it from the graph.
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Advanced Chemistry with Vernier
Using Freezing Point Depression to Find Molecular Weight
Part II Determine the Freezing Temperature of a Solution of Benzoic Acid and Lauric Acid
12. Prepare the computer for data collection.
a. From the Experiment menu, choose Store Latest Run. This stores the data so it can be
used later.
b. To hide the curve of your first data trial, click the Temperature y-axis label of the graph
and choose More…. from the dropdown list. Uncheck the Run 1 Temperature box and
.
click
13. Prepare an ice bath by adding plenty of ice to the room temperature water bath previously
used.
14. Separately weigh ~1 g of benzoic acid and ~8 g of lauric acid. Record the exact masses of the
two solids and add them to the same test tube. Repeat Steps 7–10, except you’ll lower the
molten lauric acid/benzoic acid solution into the ice bath instead.
Freezing Point
Time
15. The freezing temperature of the benzoic acid-lauric acid solution can be determined by
finding the temperature at which the mixture initially started to freeze. Unlike the behavior of
pure lauric acid, cooling of the lauric acid/benzoic acid mixture results in a gradual linear
decrease in temperature during freezing. Follow the steps below to analyze the graph.
a. Click and drag the mouse to highlight the initial part of the cooling curve where the
temperature decreases rapidly (before freezing occurred).
b. Click on the Linear Regression button, .
c. Now click and drag the mouse over the next linear region of the curve (the gently sloping
section of the curve where freezing took place).
d. Click again. The graph should now have two regression lines displayed.
e. Choose Interpolate from the Analyze menu. Move the cursor to the point of intersection of
the two lines; you will know if the cursor is over the intersection when the temperature
readings displayed in the interpolate box are the same. This is the freezing point of the
benzoic acid-lauric acid mixture.
f. Record the freezing point in your data table.
16. Print a graph showing both trials.
Advanced Chemistry with Vernier
4-3
Computer 4
DATA TABLE
Mass of lauric acid (g)
Mass of benzoic acid (g)
Freezing temperature of pure lauric acid (°C)
Freezing point of the benzoic acid-lauric acid mixture (°C)
DATA ANALYSIS
1. Using the freezing temperatures of pure lauric acid and of the benzoic acid/lauric acid
mixture, calculate the molality (m), in mol/kg, of benzoic acid in the mixture using the
formula ∆T = i × Kf × m. The value of Kf for lauric acid is 3.9°C•kg/mol, and i = 1.
2. Calculate the number of moles of benzoic acid solute, using the molality determined above
and the mass (in kg) of lauric acid solvent.
3. Calculate the experimental molecular weight of benzoic acid, in g/mol.
4. Determine the accepted molecular weight of benzoic acid from its formula, C6H5COOH.
5. Calculate the percent discrepancy between the experimental and accepted values.
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Advanced Chemistry with Vernier