Molar Mass Determination from Freezing Point Depression
Previously, compounds such as p-dichlorobenzene, biphenyl, benzil, and
naphthalene were used. As these compounds are toxic and therefore potentially
hazardous, they have been replaced with naturally occurring non-toxic fatty acids. We
will make soap from the waste from this lab so don’t toss anything down the drain.
When a nonvolatile solute is dissolved in a solvent, the solvent’s boiling point is
increased, its freezing point is decreased, and its osmotic pressure is increased. These
characteristics of a solution are termed the colligative properties. Colligative effects are
observed in many phenomena such as the melting of ice on winter roads from “salting”,
addition of antifreeze to a car’s radiator to prevent “freeze up”, and the rising of water in
trees, due to osmotic pressure.
The amount the properties of a mixture change due to colligative effects depends
on the number of moles of solute particles dissolved in the solvent and not the nature of
the particles. It is therefore imperative to know whether the dissolved solute produces one
mole of particles for every mole dissolved, or whether the solute dissociates into two,
three or more particles on dissolving. Ionic compounds when dissolved in water produce
more than one mole of particles (ions). NaCl produces two moles of ions, while CaCl2
produces three moles of particles.
The relationship between the boiling point elevation (BPE) and the freezing point
depression (FPD) to the corresponding change in temperature is given by the following
equations:
BPE : ΔTb = ikbm
FPD : ΔTf = ikf m
Where, ΔTb is the difference between the boiling point of the solution and the boiling
point of the solvent and Tf is the difference between the freezing point of the solution
and the freezing point of the solvent. The change is temperature, ΔT, is always a positive
value. The number of particles is represented by the Van’t Hoff Factor, i. In this
experiment the solute is molecular and i will equal one. The concentration of the solution
is determined in molality, m. The molal boiling point constant, kb, and the molal freezing
point constant, kf, are specific for each solvent. Some representative values are shown
below.
Solvent
water
benzene
camphor
stearic acid
kb
(°C
kg/
0.52
2.53
40.0
unk
mole)
kf
(°C
kg/
mole)
1.86
5.10
5.95
4.50
One use of colligative properties is the determination of the molecular weight of a
substance. In this experiment, the freezing point of pure stearic acid will be measured. A
weighed quantity of an unidentified sample will be added to the pure stearic acid and the
freezing point re-determined. The difference between the freezing points of pure stearic
acid and the mixture is the freezing point depression, ΔTf. Knowing the molal freezing
point constant for stearic acid, the molecular weight of the unidentified sample can be
determined by using the following equations.
Data Treatment
To accurately determine the freezing points of pure stearic acid and each of the solutions,
“cooling curves” of temperature (y-axis) versus time (x-axis) are plotted. Data where the
temperature changes by more than 0.5 °C per 30 seconds is plotted as one series and data
where the temperature changes by less than 0.5 °C per 30 seconds as a second series. The
freezing point is obtained by seeing where the temperature levels off. This can be done on
the fly by simply monitoring the temperature as it drops.
Data from the video for the Pure Stearic Acid
Time
0
30
60
90
120
150
210
240
270
300
330
360
390
420
480
Temperature
88.8
82.4
78.7
76
73.9
71.7
67.3
67
67
67.1
67.2
67.2
67.2
67.1
67
Graph of the data from the video for pure stearic acid.
Pure Stearic Acid Freezing Curve
95
Temperature (Celcius)
90
85
80
Series1
75
70
65
60
0
100
200
300
400
500
600
Time (seconds)
Procedure
You will place 20g of pure stearic acid in a test tube, melt it and determine its MP. You
will then need to add enough solute to give a change in temperature of at least 3 oC. Use 2
grams and then 2 more grams.
Report: You are ultimately trying to determine the molecular weight and identity of your
unidentified sample using the list below.
Myristic Acid
Palmitic Acid
Lauric Acid
C14H28O2
C16H32O2
C12H24O2