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Boiling Point Elevation

4-4 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 it 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…...

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