Laboratory
15.2
Effect of Solution Concentration on Freezing Point (4)
Lab Report = 145
(5) Introduction:
The addition of solute causes the freezing point of
a solvent to be depressed.
The freezing
temperature of the resulting solution can be
calculated by using the equation
∆Tƒ = Kƒ*m*i
∆Tƒ is the change in freezing point in degrees
Celsius, Kƒ is a constant called the freezing point
depression constant, m is the molality of the
solution, and i is the vant Hoff factor.
Molality (m) is defined as the number of moles of
solute per the number of kilograms of solvent,
usually kilograms of water.
When molecular solutes dissolve in a solution, they
remain as molecules. In the case of ionic solutes,
such as sodium chloride, the solid dissociates into
its component ions.
NaCl(s)  Na+(aq) + Cl-(aq)
Since the depression of the freezing point depends
on the concentration of particles in solution, the
effective molality, or concentration, is increased by
a factor equal to the number of ions produced. In
the case of sodium chloride, two ions are produced.
Therefore, for NaCl, the freezing point expression is
∆Tƒ = Kƒ*m*2
In this experiment we will compare the empirical
freezing point depression of three solutions to their
calculated theoretical values.
•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
(2) Objectives:
1. Calculate the molal freezing point depression
constant of the solvent of nonionic and ionic
solutions from freezing point depression data.
2. Demonstrate an understanding of the effects
of ionic and nonionic solutes on the freezing
points of solutions.
3. Relate the information from this experiment to
the salting of icy roads.
••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
(5) Materials:
Apparatus:
ULI w/ temperature probe
Polystyrene cup
250 mL beaker
10-mL graduated cylinder
4 18 X 150 mm test tubes
safety goggles
laboratory apron
••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
Reagents:
crushed ice
deionized water
sucrose, C12H22O11
sodium chloride, NaCl
calcium chloride dihydrate,
CaCl2.2H2O, FW=147.0
•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
Procedure:
1. Put on your safety goggles and laboratory
apron.
2. Check to make sure the ULI is plugged into
the power outlet and attached to the modem
port of the computer. Be sure the
temperature probe is plugged into Port 1 and
that the ULI power switch is on.
3. Prepare the computer for data collection by
opening Logger Pro on the Desktop. Change
the vertical axis it to –10 to 5°C. To change
the scale, click on the number you want to
change. For example, if the top of the y-axis
is 50, click on 50 and change to 5.
Laboratory 15-2
4. Change the horizontal axis to 0 to 300.
5. Hit Clrl + D to open up collection data.
6. Change the collection to 300 seconds and hit
OK
7. Number four 18 mm X 150 mm test tubes.
Pour exactly 5.00 mL of deionized water into
each.
8. To tube 2 add ≈ 3 grams of sucrose. Record
the mass to ±0.01 g.
9. To tube 3 add ≈ 0.5 grams of sodium
chloride. Record the mass to ±0.01 g.
Page 1
10. To tube 4 add ≈ 0.7 grams of calcium
chloride dihydrate, CaCl2•2H2O. Record the
mass to ±0.01 g.
11.Mix each tube thoroughly until solute is
completely dissolved.
12. Place the temperature probe into test tube
#1. Lower the test tube into the ice bath.
13. Immediately click “Collect” to begin data
collection.
14, Add about a tablespoon of salt into the ice.
Stir the mixture with the test tube until the
freezing temperature has been established.
15. Stir the water with the temperature probe
using short, quick horizontal strokes of the
test tube as the probe stays still, for the
duration of the experiment. Be careful to keep
the probe in, not above, the ice as it forms.
You are stirring the solutions by moving the
test tube, not the probe.
16. As the ice in the beaker melts, pour off water
as necessary and add ice and salt to replace
it.
17. Continue until the freezing point is
established, then click “Stop”.
18. Remove the test tube from the ice-salt-water
slush and remove the temperature probe.
19. On the displayed graph, analyze the flat part
of the curve to determine the freezing point of
water. Move the mouse pointer to the
beginning of the graph’s flat part, press the
mouse button and hold it down as you drag
across the flat part to select it. Then click
“Analyze” menu, and “Statistics.” The mean
temperature value for the selected data is
listed in the statistics box on the graph.
Record this temperature as Tf for DI water.
20. From the “Experiment” menu, choose Store
Latest Run. This stores the data so that it can
be used later. To hide the curve of your first
data run, click the Temperature vertical-axis
label of the graph, and uncheck the
Temperature box under Run 1. Click OK.
21. Rinse and place the temperature probe into
test tube #2. Lower the test tube into the
ice/salt bath.
strokes, for the duration of the experiment.
Be careful to keep the probe in, not above,
the solid as it forms.
25. As the ice in the beaker melts, pour off water
as necessary and add ice and salt to replace
it.
26. Continue until the freezing point is
established, then click “Stop”.
27. Remove the test tube from the ice-salt-water
slush and remove the temperature probe.
28. On the displayed graph, analyze the freezing
curve and extrapolate back to determine the
freezing point of sucrose/water solution. To
analyze the data, repeat step 19. Record the
freezing temperature of the solution.
29. From the “Experiment” menu, choose Store
Latest Run. This stores the data so that it can
be used later. To hide the curve of your
second data run, double click the
Temperature vertical-axis label of the graph,
and uncheck the Temperature box under Run
2. Click OK.
30. Repeat the same steps for test tube #3 and
#4.
31. Double click the Temperature vertical-axis
label of the graph. To display all temperature
runs, check the temperature boxes for Run 1,
Run 2, Run 3, and Latest boxes. Click OK.
32. Adjust the axis scales to fit your maximum
and minimum temperatures and the actual
time used.
33. Label the curves by choosing “Make
Annotation” from the Insert menu and typing
“Water” or “Sucrose soln” or “NaCl soln “ or
“CaCl2 soln” in the edit box. Then drag each
box into position near its respective curve.
34. Print a copy of the graph for you and each of
your partners. Be sure to use set up to print
in landscape and include your names.
35. Clean up all materials and wash your hands
thoroughly.
(20)36. Attach the graph, clearly labeled, to
your report.
22. Immediately click “Collect” to begin data
collection.
23. Stir the ice/salt slush with the test tube until
the freezing temperature has been
established.
24. Stir the sucrose/water solution with the
temperature probe using short, quick vertical
Laboratory 15-2
Page 2
••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
(82) Data & Calculations: Show all calculation derivations with dimensional analysis.
(25)
1 (H2O)
2
3
4
Solute
XXXX
XXXX
XXXX
XXXX
XXXX
Sucrose
NaCl
CaCl2-2H2O
Mass of
solute
Moles of
solute
Molality of
solution
i, vant Hoff
factor
Tƒ
∆Tƒ,
experiment
∆Tƒ,
theoretical
% error
XXXX
XXXX
XXXX
(15) Stamp
For each solution do the following, showing all
derivations, and fill in the data table:
(6) 5. Calculate ∆Tƒ, experiment, from your
experimental Tƒ values.
(6) 1. Calculate moles of solute.
(6) 6. Calculate ∆Tƒ, theoretical, using Kƒ, m,
and i.
(6) 2. Calculate kilograms of solvent.
(6) 3. Calculate molality of solution.
(6) 7. Calculate % error
(6) 4. Determine van’t Hoff factor. If >1, give
equation for dissociation.
•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
(10) Analysis:
(6) 1. Suppose you had a 1.00 m solution of
AlCl3. Assuming complete dissociation, what is
the freezing point of this solution?
(4) 2. Explain the difference between a 1.00
molal solution of sodium chloride and a 1.00
molar solution of sodium chloride by giving
directions for their preparation.
•••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
(12) Real World Connections:
(8) 1. Local and state highway departments use
either rock salt, NaCl, or calcium chloride,
CaCl2, to melt ice on the roads. These salts
reduce the freezing point of water, thus turning
Laboratory 15-2
the ice on the roads to slush. If you had a 50kg
bag of each salt, which would give the greatest
amount of melting? Assume that you have a
Page 3
metric ton of ice. Justify your answer by using
calculations. Clearly show those calculations.
(4) 2. Salting roads is not accepted by everyone.
What are some drawbacks to using any salt on
the roads?
(5) Conclusions:
Develop a conclusion on why freezing point
depression occurs and why it differs between
different compounds. Summarize your findings
in a clear, precise manner.
Experiment 6-1
Page 4