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.
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.
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.
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.
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.
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 –
boiling temp of pure water
boiling temp of solution
The boiling point elevation can be predicted using the equation ∆T = Kb x m x 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
change in boiling point = Kb * m * i
= 0.51 * [(4.0035 /58.5) / 0.0998 kg] * 2
= 0.6994 °C
Calculate the predicted change in boiling point, in °C for your solution. 101
Virtual ChemLab: General Chemistry, Student Lab Manual/Workbook, V. 2.5, Third Edition, by Brian F. Woodfield and Matthew C. Asplund. Published by Prentice Hall. Copyright © 2006 by Pearson Education, Inc.
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 with the actual boiling point. 99.8 + .69 = 100.49°C, Actual boiling point = 101.20°C
Virtual ChemLab: General Chemistry, Student Lab Manual/Workbook, V. 2.5, Third Edition, by Brian F. Woodfield and Matthew C. Asplund. Published by Prentice
5-1: Boyle’s Law: Pressure and Volume
Robert Boyle, a philosopher and theologian, studied the properties of gases in the 17th century. He noticed that gases behave similarly to springs; when compressed or expanded, they tend to ‘spring’ back to their original volume. He published his findings in 1662 in a monograph entitled The Spring...
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