D
ETERMINATION OF
M
OLAR
M
ASS BY
F
REEZING
-P
OINT
D
EPRESSION
|
141
Determination of Molar Mass by Freezing-Point Depression
OBJECTIVES:
•
Gain familiarity with colligative properties of nonelectrolyte solutions
•
Find the molar mass of a solute by the method of freezing-point depression
•
Evaluate the accuracy of the method by comparison to a molar mass calculated from a
molecular formula.
DISCUSSION:
When a solution forms, the freezing point of the solution is lower than the freezing point of the
pure solvent. The magnitude of this
freezing-point depression
(
∆
T
f
) depends only on the number
ratio of solvent and solute molecules (or ions) in the solution, not on their chemical identity. This
makes
∆
T
f
one of the
colligative properties
of solutions.
If we express the solution concentration as a
molality
defined so that 1
m
=
1 mol solute
1 kg solvent
⎛
⎝
⎜
⎞
⎠
⎟
, then
the freezing-point depression follows a simple direct proportionality relationship:
∆
T
f
= K
f
·m
(1)
where the proportionality constant,
K
f
, is called the molal freezing-point depression constant.
Lauric acid (the solvent in this experiment) has a reported
K
f
=
3.9 °C·kg/mol = 3.9 °C/
m
.
In this experiment, you will determine the freezing point of the pure solvent, CH
3
(CH
2
)
10
COOH
(lauric acid). You will then find the freezing point of a lauric acid solution that contains a
measured mass of solute C
6
H
5
COOH (benzoic acid) and determine the freezing-point depression.
Using the experimental
∆
T
f
value and the reported
K
f
value in equation (1) will enable you to
find the amount (in moles) of benzoic acid in your solution. This value, along with the known
mass of benzoic acid, leads to the molar mass determination.
PROCEDURE:
Part I–Freezing point of pure lauric acid
1.
Set up two water baths (one at about 80 °C and one at room temperature) using 400-mL
beakers filled to about 300 mL. Obtain a sample of pure lauric acid in a sealed test tube.
