Molar Mass by Freezing Point Depression
AP Chemistry Laboratory #5
Catalog No. AP6356
Publication No.6356A
Introduction
A procedure for determining the molar mass of a substance is very useful to chemists. The molar mass
is an important value that must be known in order to identify an unknown substance or to characterize a
newly prepared compound.
Concepts
•Molality
•Colligative properties
•Freezing point depression
Background
There are a number of ways of determining the molar mass of a substance. One of the simplest involves
finding the change in the freezing point of a solvent when an unknown substance is dissolved in it. The
change in freezing point is directly proportional to the molality of the solution. This change in freezing point
is one of several "colligative" properties of solutions-properties that depend only on the number of dissolved
particles in solution, and not on the type of particle. Other colligative properties include changes in boiling
point, vapor pressure, and osmotic pressure. Measurements of these properties can be used to find the
molar mass of solutes, also.
The molality of a solution, m, is defined as the moles of solute divided by the kilograms of solvent:
m
moles (solute)
kg (solvent)
Equation 1
Since the moles of solute is the same as the grams of solute divided by the molar mass of the solute, then:
m
g (solute)
kg (solvent)  molar mass (solute)
Equation 2
The relation of molarity to change in freezing point is:
ΔTb = kbm
Equation 3
where ΔTb is the change in freezing point of the pure substance versus the solution, kb is the freezing point
depression constant for the solvent, and m is the molality of the solution. The value of kb must be
determined for each solvent.
@ 2003 Flinn Scientific, Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Molar Mass by Freezing Point Depression Catalog No. AP6356,
from Flinn Scientific, Inc. No part of this material may be reproduced or transmitted in my form or by any means, electronic or mechanical, including, but not limited to photocopy, recording, or any
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Molar Mass by Freezing Point Depression
Page 2
Equations 2 and 3 are combined to solve for the molar mass of the solute.
Molar mas (solute) 
k f  g(solute)
kg (solvent)  T f
Equation 4
The solvent used in this experiment is a nonpolar solvent with the common name butylated hydroxytoluene. This
compound is abbreviated BHT and is frequently used as an antioxidant in foods. The IUPAC name for the
compound is 2,6-di-tert-butyl-4-methylphenol. Its structural formula is:
OH
(CH3)3C
C(CH3)3
CH3
The freezing point of BHT is approximately 70 °C. If the freezing points are determined for both the solvent and the
solution using a thermometer with minor scale divisions marked every 0.1 °C, the freezing points can be estimated
in the range: +0.02 °C.
Figure 1 shows cooling curves obtained for both a pure solvent and for a solution. Notice that super cooling occurs in
both the solvent and the solution. When super cooling occurs, the temperature falls below the freezing point until the
first crystal forms. The temperature then rises up and either stays at the freezing point, in the case of the pure
substance, or slowly falls as the solution freezes. The freezing point temperature, Tf, of the solution is extrapolated
from the graph.
@ 2003 Flinn Scientific, Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Molar Mass by Freezing Point Depression Catalog No. AP6356,
from Flinn Scientific, Inc. No part of this material may be reproduced or transmitted in my form or by any means, electronic or mechanical, including, but not limited to photocopy, recording, or any
information storage and retrieval system, without permission in writing from Flinn Scientific, Inc.
Molar Mass by Freezing Point Depression
Page 3
Experiment Overview
The purpose of this experiment is to determine the molar mass of an unknown substance by meauring the
freezing point depression of a solution of the unknown substance and BHT. The freezing point of BHT is first
determined. Even though the freezing point of butylated hydroxytoluene is known, it is necessary to determine it
with the thermometer that is used in the experiment. Thermometers can give temperature readings that are
slightly different from true values. Even if the thermometer reading is slightly off, the change in temperature should
be accurate. It is important that the same thermometer is used to determine both the freezing point temperature of
the solvent and that of the solution.
A known amount of cetyl alcohol is then added to a measured quantity of BHT. The freezing point depression of
this solution is found and the freezing point depression constant (kf) is calculated. The unknown is added to BHT,
the freezing point depression of this solution is measured, and the molar mass of the unknown is then determined.
Pre-Lab Questions
1. The following data was obtained in an experiment designed to find the molar mass of a solute by freezing point
depression.
Solvent: para-dichlorobenzene
Freezing point depression constant: 7.1 °C/m
Freezing point of pure solvent: 53.02 °C
Mass of para-dichlorobenzene: 24.80 g
Mass of unknown substance: 2.04 g
Freezing point of solution: 50.78 °C
a. Determine the freezing point depression, ∆Tf.
b. Using Equation 4, calculate the molar mass of the unknown substance.
2. What are colligative properties?
Microscale Procedure
Equipment
2, 6-Di-tert-butyl-4-methylphenol, BHT, 0.5g
Mixture of BHT and Cetyl alcohol,
CH3(CH2)14CH2OH, 0.5g
Mixture of BHT and Unknown substance, 0.5 g
Beaker, 10-mL or 50-mL
Thiele melting point tube
Bunsen burner
Capillary tubes
Cork or split rubber stopper
Balance (0.001g precision)
Stirring rod
Thermometer with 0.1 °C divisions
Small rubber band
Universal clamps
Mortar and pestle
Procedure
1. Pulverize a small amount (about 0.5 g) of BHT. Use a mortar and pestle, or use a watch glass and the bottom
of a test tube.
2. Pack the BHT in a capillary tube to a depth of about 1 cm. To get the BHT into the capillary tube, push the
open end of the capillary tube down into a small pile of BHT powder. Turn the tube so the open end is up, and
bounce the bottom of the tube on the desk top. The BHT may also be packed by holding a long piece of 6 mm
diameter glass tubing (1m in length) upright on the floor, and dropping the capillary tube down the glass
tubing so it bounces up and down a few times.
3. Obtain a small rubber band, and use it to fasten the capillary tube to a thermometer. The BHT should be level
with the bulb of the thermometer.
4. Use a universal clamp and split rubber stopper to fasten the thermometer to a ring stand.
5. Immerse the bottom of the capillary and thermometer in a Thiele melting point tube filled with water and heat
@ 2003 Flinn Scientific, Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Molar Mass by Freezing Point Depression Catalog No. AP6356,
from Flinn Scientific, Inc. No part of this material may be reproduced or transmitted in my form or by any means, electronic or mechanical, including, but not limited to photocopy, recording, or any
information storage and retrieval system, without permission in writing from Flinn Scientific, Inc.
(see Figure 4).
Molar Mass by Freezing Point Depression
Page 4
Figure 4
6. Heating can be rapid in the beginning, but as the temperature approaches the melting point, ≈ 67 °C, heat
very slowly in order to get an accurate value.
7. Record the temperature at which the BHT melts (the white powder will become clear) in your notebook.
8. Repeat the melting procedure using a new sample and capillary tube.
9. Pulverize a small amount of the mixture of BHT and cetyl alcohol that was prepared by your instructor.
Record the exact amount of each compound in the mixture.
10. Repeat steps 2-8 to determine the solution melting point.
11. Repeat steps 2-8 using the mixture of BHT and the unknown substance that was prepared by your instructor.
Record the exact amount of each compound in the mixture.
Calculations
Determine ∆Tf for the solution of cetyl alcohol and of the unknown substance in BHT. Calculate the molality
of the cetyl alcohol solution and use it to determine the value of the freezing point depression constant,Kf,for
BHT. Use the calculated value of Kf, along with the masses of the unknown solute and BHT, to find the molar
mass of the unknown solute.
Post-Lab Questions (Show all work in your notebook)
1. The following errors occurred when the above experiment was carried out. How would each affect the calculated
molar mass of the solute (too high, too low, no effect)? Explain your answers.
a. The thermometer used actually read 1.4 °C too high.
b. Some of the solvent was spilled before the solute was added.
c. Some of the solute was spilled after it was weighed and before it was added to the solvent.
d. Some of the solution was spilled after the solute and solvent were mixed but before the freezing point was
determined.
2. What was the least precise measurement in the experiment? How does this limit your significant digits?
3. Did the solutions show any evidence of super cooling?
4. Why is it advantageous to choose a solvent which has a large value for K f?
5. Explain why the pure solvent shows a level horizontal curve as solidification occurs, but the curve for the solution
slopes down ward slightly.
@ 2003 Flinn Scientific, Inc. All Rights Reserved. Reproduction permission is granted only to science teachers who have purchased Molar Mass by Freezing Point Depression Catalog No. AP6356,
from Flinn Scientific, Inc. No part of this material may be reproduced or transmitted in my form or by any means, electronic or mechanical, including, but not limited to photocopy, recording, or any
information storage and retrieval system, without permission in writing from Flinn Scientific, Inc.