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4-5:FreezingPointDepression

If youdissolve a substancesuch as ordinarytable salt (NaCl) in water, the freezing point of the water will decrease relative to the freezing point ofthe pure water. This property is used tomelt the snow or ice on roads during the winter or to make homemade ice cream. In this assignment, youwill dissolve a sample

of NaCl in water, add some ice, andthen measure the freezing point depression.

1.    Start VirtualChemLab, select Colligative Properties,and select Freezing Point Depression fromthe list of assignments. The lab will open in the Calorimetry laboratorywith a beaker containing45.00g of iceand a coffee cup calorimeter on the lab bench.  A sampleof sodiumchloride (NaCl) will also be on the balance.

2.    Click on theLab Booktoopen it. Record the mass of the sodiumchloride in the data table.  If it is too small to read,click on theBalance areato zoomin, record the mass, and then return to the laboratory.

3.    100 mL of water is already in the calorimeter. Use the densityofwater at 25^oC (0.998 g/mL) to determine the mass from the volume andrecord it in the data table. Make certainthe stirrer isOn

(you shouldbe able to see the shaft rotating). In thethermometer window, clickSave to begin recording data in the lab book.Allow 20-30seconds to obtain a baseline temperature of the water. Click the clock on the wall labeledAccelerate 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 windowsto bring themto the front andobserve the change intemperature in the graph window until it reacheszero.Drag the weigh paper fromthe balance to the calorimeter and thenpour it intothe calorimeter. Observe the change in temperature until it reaches a stableminimumand clickStopin thetemperaturewindow. A data link icon will

appear in the lab book. Click the data link icon and record thelowest temperature after adding the salt

in the data table. (Remember that the watermayhave begun to warmback up.)

5.    If you want torepeat the experiment, click on the reddisposal bucket to clear the lab, click onthe Stockroom, click on the clipboard, and select Preset Experiment #1, Freezing Point Depression – NaCl.

DataTable

6.    The freezing pointdepression can be predicted using the equation   ∆T=Kf x m x i, where  ∆T is the

change in freezing point, iis the number of ions inthesolutionpermole of  dissolved NaCl (i = 2), m

is themolality of the solution, and Kf   isthe molal freezing point constant for water which is

1.86^oC/m.

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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.