Chapter 12: Physical Properties of Solutions
LAB…Colligative Properties: Freezing Point Depression
Background:
The physical properties of solutions that depend on the number of
dissolved solute particles and not their specific type are known as
colligative properties. There include freezing point depression, osmotic
pressure, and boiling point elevation.
Freezing occurs when a solute is added to a solvent producing a solution
having lower freezing point temperature than the pure solvent. The
temperature decreases by an amount Tf given by the following
formula…
Tf =K fcm
when K f is the freezing point depression constant and cm is the molal
concentration of the solution in moles of solute particles per kilogram of
solvent (moles/kg). One way to understand the freezing point depression
effect is to consider the solute particles as interfering or standing between
the solvent particles. With greater space between solvent particles,
intermolecular forces are weaker. Consequently, lower temperatures are
required to make it possible for solvent particles to approach each other
and form the solid.
Introduction:
In this lab the changes in freezing point behavior of solutions in which tbutyl alcohol (2-methyl-2-propanol) is the solvent will be explored.
**It is important to note the identity of solute particles isn’t specified.
That is, a 0.50 m C6H12O6 solution should have the same freezing
point as an aqueous 0.25 m NaCl solution, since each formula unit
of NaCl provides two ions in solution.
In this lab, the freezing point of t-butyl alcohol will be determined by
cooling it in cold water. Then a solute will be added to the alcohol and
the freezing point of the solution will be measured. After determining the
freezing point and actual Tf of the solution, and the masses of solute and
solvent, the K f of the solvent should then be determined.
Materials:
 Logger Pro
 Excel
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Temperature Probe
Large Test Tube
25mL graduated cylinder
T-butyl Alcohol
Scale
2: 400mL beakers
Ice
Hot Place
Ring Stand and Clamp
2: Copper Stirrers
Procedures:
1. Connect a temperature to LabPro interface and launch Logger Pro
and Excel applications on computer. Set Logger Pro to “Time
Based” mode with an experiment length of 10 minutes and a
sampling rate of 10 samples/minute
2. Measure and record the mass of a clean, dry large test tube (**it is
extremely important the test tube is clean and dry!!)
3. Using a graduate cylinder, pour 20 mL of t-butyl alcohol into the test
tube and measure the new mass. Using basic arithmetic, find the
mass of the t-butyl alcohol. Record data in excel spread sheet or
lab notebook.
4. Prepare an ice water bath in a 400-mL beaker. Prepare hot water
bath in a 400-mL beaker using a hot plate (temperature should be
40-50C).
5. Clamp test tube to ring stand and insert a clean, dry copper stirrer
into the test tube and clamp temperature probe into the test tube.
(**make sure that the probe does not touch the walls of the test
tube and yet is still well-immersed in the liquid and copper stirrer
should surround the probe).
6. When you are ready to begin data collection click on the “start”
button and quickly lower the test tube into the cold water bath.
Continuously stir the test tube inside the beaker in an up/down
motion. (**make sure the probe stays in the water and does not hit
the wall of the test tube). Continue stirring and collecting data until
temperature levels off. Once a constant temeperature is obtained
remove the test tube from the ice bath and place it in the hot
water bath. This with then thaw the solid.
7. Save data collected.
8. Repeat steps 4-6, so multiple trials of data may be collected.
9. After two trials remove test tube from the clamp and dry thoroughly.
Next, record the mass of the test tube and contents and calculate
the mass of remaining t-butyl alcohol by using the mass if the test
tube recorded previously.
10. Dispose of the solution in waste beaker and clean all equipment.
Calculations and Questions:
1. Calculate the molality for each trial of the solutions.
2. Calculate the freezing point depression for each trial of the
solutions.
3. Using the molality and freezing point depressions, calculate the
freezing point depression constant for teach trial of solutions.
4. Find the average value for the Kf.
5. Assuming the values obtained for the freezing point and Kf of t-butyl
alcohol are correct, find the molecular weight of the substance that
yielded a freezing point of 18.2C when 1.06 g of the substance was
dissolved in 18.36 g of t-butyl alcohol.
6. Melting point temperatures are frequently used to help identify
unknown solids and determine their purity. How would the melting
point of a pure solid sample compared to that of the same solid
contaminated by a solid impurity? Explain.