Boiling Point Elevation
VCL 13-4: Boiling Point Elevation
If you dissolve a substance such as ordinary table salt (NaCl) in water, the boiling point of the water will
increase relative to the boiling point of the pure water. In this assignment, you will dissolve a sample of
NaCl in water and then measure the boiling point elevation for the solution.
1. Start Virtual ChemLab and select Boiling Point Elevation from the list of assignments. The lab will
open in the Calorimetry laboratory with a calorimeter on the lab bench and a sample of sodium
chloride (NaCl) on the balance.
2. 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 reading, 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 in the data table. Make certain the stirrer is On (you
should be able to see the shaft rotating). Click on the green heater light on the control panel to turn on
the heater and begin heating the water. Click the clock on the wall labeled Accelerate to accelerate the
laboratory time if necessary.
4. Observe the temperature until the first appearance of steam comes from the calorimeter. Immediately
click the red light on the heater to turn it off and then record the temperature as the boiling point of
pure water in the data table. Letting the water boil will decrease the mass of the water present in the
calorimeter. Note that the boiling point may be different than 100°C if the atmospheric pressure is not
760 Torr. The current atmospheric pressure for the day can be checked by selecting Pressure on the
LED meter on the wall.
5. Drag the weigh paper to the calorimeter and add the NaCl. Wait 30 seconds for the salt to dissolve
and then turn on the heater. When steam first appears observe and record the temperature in the data
table.
6. 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 #2, Boiling Point Elevation – NaCl.
Data Table
mass NaCl
mass water
boiling temp of pure water
boiling temp of solution
7. The boiling point elevation can be predicted using the equation ∆T = K b × m × i , where ∆T is the
change in boiling 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 Kb is the molal boiling point constant for water which is 0.51°C/m.
Calculate the predicted change in boiling point, in EC for your solution.
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Chapter 13
8. The change in boiling point must be added to the boiling point of pure water in your experiment in
order to compare the predicted boiling point with the actual boiling point.
What is the calculated boiling point of the solution? Compare this to the actual boiling point.
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