Molar Mass Determination by Freezing Point
VCL 13-7: Molar Mass Determination by Freezing Point Depression
If you dissolve a substance such as ordinary table salt (NaCl) in water, the freezing point of the water will
decrease relative to the freezing point of the pure water. You can use this property to calculate the molar
mass of an unknown. In this assignment, you will dissolve a sample of NaCl in water, measure the
freezing point depression for the solution, and then calculate the molar mass for NaCl as if it were an
unknown.
1. Start Virtual ChemLab and select Freezing Point Depression from the list of assignments. The lab
will open in the Calorimetry laboratory with a beaker containing 45.00 g of ice and a coffee cup
calorimeter on the lab bench. A sample of sodium chloride (NaCl) will also be on the balance.
2. Click on the Lab Book to open it. Record the mass of the sodium chloride in the data table. If it is too
small to read, click on the Balance area to zoom in, record the mass, and then return to the laboratory.
3. 100 mL of water is already in the calorimeter. Use the density of water at 25°C (0.998 g/mL) to
determine the mass from the volume and record it in the data table. Make certain the stirrer is On
(you should be able to see the shaft rotating). Click the thermometer window to bring it to the front
and click Save to begin recording data in the lab book. Allow 20-30 seconds to obtain a baseline
temperature of the water. Click the clock on the wall labeled Accelerate to accelerate the laboratory
time if necessary.
4. Drag the beaker of ice until it snaps into place above the calorimeter and then pour the ice into the
calorimeter. Click the thermometer and graph windows to bring them to the front again and observe
the change in temperature in the graph window until it reaches zero. Drag the weigh paper from the
balance to the calorimeter and then pour it into the calorimeter. Observe the change in temperature
until it reaches a stable minimum and click Stop in the temperature window. A blue data link will
appear in the lab book. Click the blue data link and record the lowest temperature after adding the salt
in the data table. (Remember that the water may have begun to warm back up.)
5. If you want to repeat the experiment, click on the red disposal bucket to clear the lab, click on the
Stockroom, click on the clipboard, and select Preset Experiment #1, Freezing Point Depression –
NaCl.
Data Table
mass NaCl
mass water
mass ice
mass water + ice
minimum temperature
6. What is the freezing point depression caused by adding NaCl to the water?
7. The freezing point depression can be calculated using the equation ∆T = K f × m × i , where ∆T is the
change in freezing point, i is the number of ions in the solution per mole of dissolved NaCl (i = 2), m
is the molality of the solution, and Kf is the molal freezing point constant for water which is
1.86°C/m.
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Chapter 13
Using this equation and the data recorded in the data table, calculate the molar mass for NaCl and
compare it with the actual value.
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