Experiment 9: Freezing Point Depression Safety Hazards
Experiment 9: Freezing Point Depression Safety Hazards
Experiment 9: Freezing Point Depression Safety Hazards
Some properties of solutions do not depend on the nature of the solute: they
only depend on the number of solute particles (ions or molecules) relative to Background
the number of solvent particles. We call these properties colligative
properties. In this experiment we will focus on one particular colligative
property, the freezing point depression.
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BC Chemistry 162 Laboratory Manual
Cooling curve
(solution)
where i is the van’t Hoff factor and represents the number of ions that
would be produced after complete dissociation of the electrolyte in a very
dilute solution (ideal conditions).
Procedure
Prepare the computer for data collection by opening experiment 15 in
LoggerPro from the folder Chemistry with Computers. The vertical axis of
the graph has a temperature scale from 0°C to 100°C. The horizontal axis has
time scaled form 0 to 10 minutes.
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BC Chemistry 162 Laboratory Manual
9. Determine the freezing point of the solution using the computer graph.
Again, as explained in the background section, you will need to interpolate to
accurately determine the freezing temperature.
10. Remove the temperature probe.
11. Melt the frozen solution under the tap’s warm water.
12. Dispose of the sample in the provided chemical waste container. Do NOT
dispose of this sample anywhere else.
1. Calculate the masses of the solvent and/or solute present in each of the three
samples.
2. Calculate the freezing point depression (|ΔTf|) for the two solutions.
3. Calculate the experimental value of the molality (moles of solute per kg solvent)
of the two solutions based on the |ΔTf| for the two solutions.
4. Calculate the experimental value for the moles of solute present in each solution.
5. Calculate the molar mass (grams per mole) of the unknown solute.
6. Calculate the average molar mass of the unknown solute based on the values
obtained from the two solutions.
Procedure
In this experiment you will use two different solutions of CaCl2 to determine
the van’t Hoff factor, i, using the known Kf value for water, 1.86 °C ·kg/mol.
a. Using a 10mL pipet, add 30 mL of 0.5 m CaCl2 solution to a large test tube
and stopper with a 2 hole stopper.
b. Insert the temperature probe through the large hole of the stopper.
c. Insert the metal stirrer through the thin hole of the stopper.
d. Place the tube in the rock-salt ice bath and start collecting the data.
e. While the data is being collected stir the sample gently with an up and
down motion to avoid supercooling.
f. You can stop stirring once the sample starts freezing.
g. Collect data for 8-10 minutes.
h. Determine the freezing point of the solution using the computer graph.
Enlarge the y-axis (cursor on y-axis; click on ~ symbol and drag) to better
determine the freezing temperature. As explained in the background section
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BC Chemistry 162 Laboratory Manual
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BC Chemistry 162 Laboratory Manual
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BC Chemistry 162 Laboratory Manual Name ___________________________________ Section _____
Report Sheets
Part 1
Record the following information to demonstrate your successful completion
of the experimental objectives for Part 1.
Part 2
Record the following information to demonstrate your successful completion
of the experimental objectives for Part 2.
Follow-up questions
1. Why is molality used instead of molarity in the context of colligative properties?
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BC Chemistry 162 Laboratory Manual
2. Assume that in the acetic acid experiment you obtained the following values:
Tf (acetic acid) = 16.1°C Tf (with 3 mL of unknown) = 11.3°C
b. Your instructor shows you that you made a mistake in the interpolation and that the correct
value is Tf (with 3 mL of unknown) = 11.0 °C. Calculate the new molar mass of the unknown and
compare this value with the molar mass determined based on Tf = 11.3 °C.
2. What do you think are the major sources of error in the determination of the molar mass of the
unknown?
3. What is the theoretical value of the van’t Hoff factor, i, for CaCl2? How does your experimental
value compare to the theoretical value? What does this suggest is happening at the molecular level
in the solution?
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BC Chemistry 162 Laboratory Manual Name ___________________________________ Section _____
2. Calculate the molality of a solution prepared by dissolving 78.0 grams of butanone (C4H8O) in
800 mL of acetic acid. The density of acetic acid is 1.049 g/mL.
3. You dissolve 93.24 g of an unknown solute in 1000 g of water and obtain | ΔTf | = 2.34°C.
a. What is the molality of the solution if Kf is 1.86 °C ·kg/mol for water?
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