Academic Chemistry Lab: Freezing Point Depression and Molar Mass Determination
Background: There are two parts to a solution. The substance being dissolved is the solute. The fluid
that the solute is being dissolved in is the solvent. Colligative properties of solutions depend on the
quantity of solute dissolved in a solvent rather than the identity of the solute. The freezing point of a
solvent is lowered when a solute is added. This is called freezing point depression. The freezing point
depression can be used to calculate the molar mass of the solute.
For example, if sugar is added to water, the amount of water that can escape form the surface of the liquid
decreases as compared to pure water. The larger sugar molecules interfere with the vaporization of the
water molecules. The vapor pressure above such a solution would be lower than that of the pure water at
the same temperature. Molecules of solutes such as sugar, block the surface of the solvent, thus
preventing as many molecules from evaporating. The decrease in vapor pressure results in an increase in
the boiling point and a decrease in the freezing point.
In this study you will determine the depressed freezing point of naphthalene when an unknown solute is
added. You will then determine the molar mass of the solute. The decrease in freezing point (∆Tf) when
a solute is dissolved is proportional to the concentration of solute. The solvent has a freezing point
depression constant Kf. This represents the number of degrees the freezing point will change if 1.00 mole
of solute is added to 1.00 kg of solvent. Kf is different for different solvents. Kf for water is 1.86 ºC
kg/mole, whereas Kf for benzene is 5.12 ºC kg/mole.
The measurement of freezing point depression (∆Tf) is routinely used for the determination of the molar
mass of unknown solutes. The following equation can be used to calculate molar mass.
Molar Mass =
(Kf)( gsolute)
(∆Tf )(kgsolvent)
For example: The freezing point depression constant for benzene is 5.12 ºC kg/mole. When 1.08g of
unknown solute is dissolved in 10.02g of benzene (the solvent), the solution freezes 4.60 ºC lower than
pure benzene. Calculate the molar mass of the unknown solute.
(5.12 ºC kg/mol)(1.08g)
Molar mass = ------------------------------- = 120g/mole
(4.60ºC)(0.01002kg)
Materials
13x100mm test tube
Hot plate
Lauric acid
one hole split stopper
Computer
unknown solute
thermometer probe
Vernier software
test tube clamp
two 100mL beakers
beaker tongs
ring stand
Name _____________________________________ Period _______ (10 points)
Lab Partner: ________________________________
Academic Chemistry Lab: Freezing Point Depression and Molar Mass Determination
Procedure:
1. Place two 100mL beakers about ¾ full of water on a hot plate and heat to boiling.
2. Clean and dry a 13x100mm test tube. Mass of this test tube is _________________________.
3. Fill the test tube w/ ~ 8grams lauric acid and record the mass. ________________________
4. On a folded paper, mass approximately 1g of unknown solute to 0.001g. Also add this to the test tube
and record the mass. ___________________________
5. Clamp the test tube in the hot water bath and allow the contents to melt.
6. Plug the temperature probe into Channel 1 of the LabPro interface. Connect the LabPro to the
computer using the proper cable.
7. Modify Logger Pro to collect data for 600 seconds (10 minutes). This can be done by selecting
experiment/data collection.
8. When all the solution has melted (~ 60°C), use tongs to carefully remove the beaker and test tube
from the hot plate, insert the temperature probe and stir. Using a split 1 one holed stopper, adjust the
temperature probe so it does not touch the walls of the test tube. Begin collecting data, and allow the
water and test tube to slowly cool.
9. The freezing point of this solution can be determined by finding the y-coordinate at the top of the
small “blip.” This is also the point where the solution starts to cloud over. If you are satisfied with
you data print a copy of this graph for each member of your lab team. If your data is poor, reinsert the
test tube in the second boiling beaker to melt the sample again and cool a second time.
10. To clean-up: place the test tube in the second beaker of boiling water. Once the liquid has melted,
remove the probe and place it directly in the beaker of boiling water. Wipe the probe off while it is
still hot and return it to storage. Quickly pour the test tube of hot solution into the waste jug. Now
completely fill this test tube with water and place it in the beaker of boiling water that is in the hood.
DATA TABLE (1 point each block)
smooth calculation for molar mass solute (2 pts):
Mass Lauric acid
Mass unknown solute
Freezing Point of pure Lauric acid
44.0 ºC
Freezing point of solution
∆Tf
(change in temperature)
Kf for Lauric acid
3.9 ºC kg
mol
Note: Temperature vs time graph is worth 2 points.