Determination of an unknown by freezing point depression
Introduction
Colligative properties are properties of a solution that depend on the collective effects of solute
particles in the solution. These properties include the elevation of the boiling point of a
solution compared to the pure solvent, the depression of the freezing point of a solution as
compared to the pure solvent, reduction of the vapor pressure and the way osmotic pressure
controls osmosis. The colligative properties of a solution depend on the quantity of the solute
present in the solution and not on the chemical nature of the solute. Remember in a solution,
which can be a solid, liquid, or a gas, the component that is in the greater quantity is the
solvent and the other components of the solution are called the solutes. Colligative properties
occur because of the intermolecular forces that ate operating between the solute and solvent
in the solution. In this lab we will use the fact that a solution has a lower freezing point as
compared to the pure solvent to determine the molecular weight of the solute in the solution.
Using the colligative properties of solutions to experimentally determine the molar mass of the
solute added to a solution the identity of the unknown solute can be determined. The decrease
in the freezing point is directly proportional to the concentration of the solution expressed in
molality. Or stated another way, the freezing point change between the pure solvent and the
solvent in solution is proportional to the molality of solute in solution where molality is equal to
the number of moles of solute particles per kilogram of solvent.
∆𝑇𝑓 = 𝑘𝑓 𝑚
Here, ΔTf stands for the change in freezing point between the freezing point between the
freezing point of the solution (Tf) and the freezing point of the pure solvent, and kf is the molal
freezing point depression constant, a proportionality constant, which is different for each
solvent.
In this experiment you will determine the freezing point of pure benzophenone and then
measure the change in the freezing point when an unknown solute is added to the pure
benzophenone. Using the following set of equations, the identity of the unknown can be
determined by calculating its molecular weight and comparing its molecular weight to the
compounds listed in Table 1.
Substance
Water
t-butanol
Cyclohexane
Naphthalene
Acetic acid
Camphor
Benzoic acid
Benzophenone
Benzene
MW (g/mol)
18
74
84
128
60
152
122
182
78
F.P. (°C)
0.0
25.5
6.5
80.2
16.6
179
122.4
48.1
5.5
Kf (°C/m)
1.86
9.1
20.1
6.9
3.9
39.7
9.80
5.12
B.P. (°C)
100
Kb (°C/m)
0.52
80.7
217.7
118.3
204
249.2
2.79
5.80
3.07
5.61
80.1
2.53
∆𝑇𝑓 = 𝑘𝑓 𝑚
𝑚=
𝑚𝑜𝑙𝑒𝑠𝑠𝑜𝑙𝑢𝑡𝑒
𝑘𝑔𝑠𝑜𝑙𝑣𝑒𝑛𝑡
𝑚𝑜𝑙𝑒𝑠𝑠𝑜𝑙𝑢𝑡𝑒 =
𝑔𝑠𝑜𝑙𝑢𝑡𝑒
𝑀𝑊
Start by solving for molality using the first equation, you will have measured ΔT. Then solve for
the moles of the unknown using the second equation and finally solve for the molecular weight
of the unknown using the third equation.
Experimental Setup
Place a beaker with 400 mL of water on a hotplate. Heat the water to between 60 – 70 °C.
Temperature needs to be higher than 55 °C, but the exact temperature is not important, and
you do not need a boiling water bath. Place an 8 inch test tube into the water bath. Make sure
the test tube is not touching the bottom of the water bath; you can do this by clamping the test
tube a ring stand. Get a two-hole stopper. Place a thermometer in one hole and the metal
stirring rod in the other hole. Place the stopper in the test tube.
Experimental Procedure
Weigh out a 10g sample of benzophenone and record the actual mass. Add the benzophenone
to the test tube. Wait for the sample to melt. Remove the test tube from the water bath.
Watch closely for the first sign of crystallization. You will need to stir periodically as the
temperature is decreasing. Record the temperature of the first visible crystallization. Place the
test tube back in the water bath and repeat the procedure. This is the freezing point of the
pure solvent. You must have at least two trials. Report the average freezing point.
Weigh out a 1g sample of the unknown assigned to your group. Add this to the test tube
containing the benzophenone. Allow the mixture to melt, stir the solution to insure that the
entire unknown has dissolved in the benzophenone. Remove the solution from the water bath,
continue stirring the solution, and watch for crystallization. Record the temperature of the first
crystallization. This is the freezing point of the solution. Repeat. You should have two trials.
Add a second 1g sample of the unknown to the solution in the test tube. Repeat the procedure
of melting and recrystallization. Record the temperature of crystallization. Repeat. You should
have two trials.
Calculate the molecular weight of the unknown sample for trial 1 and trial 2. Identify the
identity of the unknown based on the information found in Table 1.
Clean Up
To clean your test tube, first melt the solution in the hot water bath. Pour the liquid solution
into the benzophenone waste container. Do not put pure benzophenone or the benzophenone
solution down the drain. Wipe the stirring wire and thermometer with a paper towel that has
been wetted with acetone. Put the used paper towel in bin in the hood. Rinse the test tube
with a small amount (2 to 5 mL) of acetone. Pour this into the waste container. Repeat the
acetone rinse.