OUR LADY OF THE ROSARY COLLEGE
CHEMISTRY PRACTICAL MANUAL
 Advanced Level
 Experiment should be carried out under the supervision of a Chemistry teacher
 Full report is required.
Division of work: Two students per group
Fractional distillation of an ideal solution
Introduction
A solution formed by two components is said to be an ideal solution if
the vapour pressure of this solution can be predicted by Raoult's:
P  1 P1   2 P2
(1)
where P1, P2, are partial vapour pressures of component 1and component 2
P is the vapour pressure of the solution.
 1 ,  2 are the mole fractions of component 1, 2 in the liquid mixture.
The mixture of methylbenzene and benzene is a good example. The
intermolecular forces in benzene, methylbenzene and their mixture are
almost the same. Hence the mixture obey Raoult's law.
It is very difficult to measure saturated vapour pressure in school
laboratory; instead temperature (here, boiling point) is much easier to be
determined by using a thermometer. Therefore, boiling points of solutions of
benzene and methylbenzene are measured. The temperatures of the distillate
are measured. A boiling point-composition diagram can be constructed.
Reagent required:
Benzene (50 mL), Toluene (50 mL), anti-bumping stone
Apparatus required
50 cm3 round-bottomed flask, flask-head adapter, stirrer gland for thermometer, Liebig
condenser, Thermometer covering range 50-110 oC, 10 cm3 measuring cylinders, Boiling tubes
Procedure
1. Place 12.0 cm3 of methylbenzene in a 50 cm3 round-bottomed flask, add a few piece of
pumice and set up the apparatus as shown in the right figure.
2. Determine the boiling-point of the methylbenzene, by warming the flask over a flame just
large enough to cause the liquid to boil.
3. Record the temperature shown on the thermometer, as soon as it becomes steady, and
estimate the possible error in the determination of the boiling-point.
4. Remove the flame to a safe distance, allow the contents to cool and raise the thermometer
so that the bulb is opposite the side-arm of the adapter as shown in figure 2.
5. Reheat the flask with a flame just large enough to maintain a ring of condensing vapour
about 2 cm above the thermometer bulb. Record the steady temperature shown on the
thermometer and estimate the possible error in this determination.
6. Add 3.0 cm3 of benzene to the methylbenzene in the flask, ensure that the two liquids are
mixed and repeat the procedure as described above for methylbenzene. Record the results.
7. Complete the experiment by following the same procedure for each of the mixtures. In the
case of mixture 5, add the benzene first and determine the results for pure benzene before
adding methylbenzene.
Data and Results
Mixture number
Volume of benzene (cm3)
Volume of methylbenzene (cm3)
Boiling point of original mixture
(oC)
Boiling point of distillate (oC)
% volume of benzene in original
mixture
% volume of benzene in
distillate (%)
1
2
3.0 6.0
12.0 9.0
30
54
3
4
7.5
7.5
9.0
6.0
66
76
5
6
12.0 Pure
3.0 benzene
7
Pure
methylbenzene
92
Questions for discussion
1. What can you say if you compare the boiling point of each distillate and of the original mixture
2.
3.
4.
5.
6.
7.
8.
from which it came?
Compare the % volume of benzene in the distillate and that in the solution.
Plot a curve of boiling-point of the original mixture (and the pure liquids) against the % by
volume of benzene.
On the same graph, plot another curve of the temperature at which each distillate is produced
(i.e. the boiling point of the mixture from which it was obtained) against its composition,
expressed as % by volume of benzene.
What can you say about the boiling point of mixtures of benzene and methylbenzene relative to
the values for the pure liquids?
Does the mixture behave as an 'ideal solution'?
Explain the term 'ideal solution' on the basis of molecular behaviour.
Using the graph you have drawn, explain what happens if a mixture of equal volumes of
benzene and methylbenzene is distilled, and the distillate is then re-distilled.
9. How many successive distillations (theoretical plates) would have to be performed to produce a
sample of benzene which is more than 95% in your phase diagram?
END