4-5: 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. This property is used to melt the snow or ice on roads during the winter or to make homemade ice cream. In this assignment, you will dissolve a sample of NaCl in water, add some ice, and then measure the freezing point depression. 1. Start Virtual ChemLab, select Colligative Properties, 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 25o 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). In the thermometer window, 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 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 data link icon will appear in the lab book. Click the data link icon 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 3.9555g mass water 99.8g mass ice mass water + ice minimum temperature 6. The freezing point depression can be predicted using the equation ?T=K f x m x 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 1.86oC/m. 107 Virtual ChemLab: General Chemistry, Student Lab Manual/Workbook, v4.5, Third Edition, by Brian F. Woodfield and Matthew C. Asplund. Published by Prentice Hall. Copyright © 2013 by Pearson Education, Inc. Colligative Properties Calculate the predicted change in freezing point for your solution. 7. The change in freezing point must be subtracted from the freezing point of pure water, which is 0.0 oC, in order to compare the predicted freezing point with the actual freezing point. What is the calculated freezing point of the solution? Compare this to the actual freezing point.