# Mandatory Experiment 3.1

```Mandatory Experiment 3.1
Estimation of the relative molecular mass, Mr, of a volatile
liquid
Student Material
Theory
According to the equation of state of an ideal gas, the relationship between the pressure,
P, volume occupied, V, temperature, T, and number of moles, n, of an ideal gas is:
PV = nRT
where R is a constant called the gas constant. Measuring pressure in Pa, volume in m3,
and temperature in Kelvins, the value of the gas constant is 8.314 J K-1 mol-1.
Thus if the pressure, volume and temperature of the vapour of a sample of a volatile
liquid are measured, the number of moles of it present can be calculated.
If the mass of the sample is known then the relative molecular mass, Mr, can then be
calculated from the relationship between mass, m, relative molecular mass, Mr, and the
number of moles, n:
n = m/Mr
In this experiment a small amount of a volatile liquid is allowed to vaporise by placing it
in a container surrounded by hot water. The temperature of the water is measured, and
therefore the temperature of the vapour is thus known. The vapour is under atmospheric
pressure and the volume it occupies at this pressure is recorded. The mass of the vapour
is also measured. The data is used to calculate the number of moles of vapour and hence
the value of the relative molecular mass, Mr, using the two formulas above.
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Method 1 – using a conical flask
Chemicals and Apparatus
Sample of propanone
(or unknown sample provided)
Water
600 cm3 beaker (into which the 250 cm3 conical flask can be easily fitted)
Aluminium foil
Clamp
Dropping pipette
Pin
Rubber band
Bunsen burner (or hotplate)
Tripod and Gauze
Thermometer
Barometer
Electronic balance
Procedure
1. Two-thirds fill the beaker with water, place on tripod stand and heat to almost
boiling with the Bunsen burner. Control the flame so that the temperature remains
at 95 0C.
2. Cut a circle of aluminium foil large enough to cover the mouth of the conical
flask and fold down a little around the sides of the flask.
3. Find the total mass of the clean dry conical flask, the aluminium foil and the
rubber band.
4. Using a dropping pipette, add 3 – 4 cm3 of the volatile liquid to the flask. You
need not worry about the exact quantity at this stage because some of this liquid
will be lost as a vapour during the experiment.
5. Cover the mouth of the flask with the aluminium foil. Hold in place tightly with
the rubber band so that no vapour can escape between the foil and the glass. With
the pin, prick one small hole in the centre of the aluminium foil cap. Attach the
clamp to the neck of the flask.
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6. Carefully immerse the conical flask into the boiling water (Fig. 1). Holding the
clamp, move the flask up and down periodically to check the liquid level in the
Fig. 1
7. All of the volatile liquid vaporises and some of it will escape out through the hole
in the cap until the pressure inside the flask is equal to atmospheric pressure.
When the flask appears to be empty (i.e. all the liquid appears to have
evaporated), this stage has been reached. Immediately and with care, remove the
flask from the beaker. Record the exact temperature of the hot water.
8. Record the value of atmospheric pressure using the barometer.
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9. Allow the flask to cool. Then, thoroughly dry the outside of the flask, including
the foil. You may now notice that there is a small quantity of liquid in the flask.
This is the volatile liquid, which has cooled and condensed. Find the mass of the
flask, cap, rubber band and contents. By subtraction of the first mass recorded, the
mass of the vapour, which occupied the flask at the temperature of the boiling
water, is now known.
10. Remove the cap and rubber band. Find the volume of the flask by completely
filling it with water, and then transferring all of the liquid from it to graduated
cylinders. Record the volume of the liquid transferred.
11. Calculate the value of the relative molecular mass, Mr, of the volatile liquid.
Table of Results
(Note that 760 mmHg = 101325 Pa)
Mass of flask, cap and rubber band
g
Mass of condensed vapour, flask, cap and
rubber band
Mass of condensed vapour
g
Atmospheric pressure
mmHg
Atmospheric pressure
Pa
g
Temperature of boiling water
0
Temperature of boiling water
K
cm3
m3
C
Gas Constant, R = 8.314 J K-1 mol-1
4
Method 2 – using a gas syringe
Chemicals and Apparatus
Sample of propanone
(or unknown sample provided)
Water
100 cm3 gas syringe, heat resistant
Self-sealing rubber cap for gas syringe
5 cm3 hypodermic syringe and needle
Gas syringe heater (electrical)
Thermometer
Barometer
Electronic balance
Procedure
Fig. 2
1. Fit the gas syringe and the thermometer into the heater. Draw about 5 cm3 of air
into the gas syringe and seal with the rubber cap. Switch on the electrical syringe
oven set to 100 0C and allow it to heat up. Allow time for the temperature to
equilibrate inside the heater.
2. Record the volume of hot air in the gas syringe.
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3. Draw 0.2 cm3 of the volatile liquid into the hypodermic syringe via the needle.
Find the mass of the syringe, needle and contents.
4. Inject the contents of the hypodermic syringe into the gas syringe through the
rubber cap, taking care to deposit the liquid into the large cylinder of the syringe.
Withdraw the needle and the cap reseals. Ensure that the narrow neck (stem) of
the syringe is inside the oven, and that only the rubber cap protrudes. The volatile
liquid vaporises and expands inside the gas syringe pushing on the plunger of the
gas syringe. When the pressure inside the syringe is equal to the atmospheric
pressure outside, the plunger comes to rest.
5. Find the mass of the hypodermic syringe and needle after the injection. Subtract
from the first reading of mass to find the mass of liquid vaporised.
6. Record the volume of hot air and vapour of the volatile liquid in the gas syringe.
By subtraction, find the volume of the vapour of the volatile liquid.
7. Record the temperature inside the heater.
8. Record the value of atmospheric pressure using the barometer.
9. Calculate the value of the relative molecular mass, Mr, of the volatile liquid.
Table of Results
(Note that 760 mmHg = 101325 Pa)
Mass of syringe, needle and contents
before injection
Mass of syringe, needle and contents after
injection
Mass of vapour
Atmospheric pressure
g
Atmospheric pressure
Temperature inside heater
Temperature of vapour
Volume of heated air
Volume of heated air and vapour
Volume of vapour
Volume of vapour
Gas Constant, R = 8.314 J K-1 mol-1
Pa
0
C
K
cm3
cm3
cm3
m3
g
g
mmHg
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Questions relating to the experiments
1. Why is it necessary that the liquid used in this experiment is volatile?
2. Name another technique for measuring relative molecular mass.
3. Since the vapour is not an ideal gas, which quantity measured in the experiment is
most likely to introduce inaccuracy in the result, and why?
4. If a small drop of water were present in the flask used in Method 1 or the gas
syringe used in Method 2, how would this affect the results?
5. From your results calculate the density of the vapour of the volatile liquid at the
temperature of boiling water.
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Teacher Material



In method 1, cooling of the flask and its contents may be speeded up using tap
water, while making sure that only the sides of the flask and not the foil get wet.
It is advisable to have a supply of hot water available at the start of this
experiment, as heating of the water can take quite a while.
A steam jacket may be used in conjunction with a gas syringe (Fig. 3) in method 2
in place of the electrical heating method. If using a steam jacket, set up a steam
generator and fill it with water. Connect the steam delivery tube to the inlet pipe
of the syringe heater and allow the outlet to drain at a sink. Heat the water to
boiling in the steam generator and allow time for the temperature to equilibrate
inside the heater. Due to condensation on the inner surface of the steam jacket it
can be difficult at times to read the volume in the gas syringe. Should this happen
it may be necessary to rotate the steam jacket using heat resistant gloves in order
to clear the surface and take a reading. Great care is needed because of the
dangers associated with the steam. The steam heating method is suitable primarily
for a teacher demonstration.
Fig. 3

In method 2, if the electrical heating method is being used, quite an amount of
time elapses before the temperature equilibrates at the desired temperature.
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
The electrical heating method is suitable primarily for a teacher demonstration, or
as part of a “circus” of student experiments.
Extension Work
Other volatile liquids could be used as well as propanone. Ethanol and methanol, for
example, are sufficiently volatile, although in the first case it would be important to carry
out the experiment in a well-ventilated laboratory, while in the second, a fume cupboard
should be used.
Molecular masses in research and industry are determined where possible with great
precision using the mass spectrometer. The method described here is mostly of interest
only from a theoretical point of view.
Preparation of Reagents
None.
Quantities needed per working group
3 – 4 cm3 of propanone
2 dm3 water
Safety considerations



The usual precautions when handling glassware should be observed.
Safety glasses and gloves must be worn.
Care should be taken when attaching the needle to the hypodermic syringe and
when using the two afterwards.

Chemical hazard notes
Propanone is highly flammable. When proceeding by Method 1 or using the steam
generator in Method 2, students are using a highly flammable liquid and a naked flame.
The propanone should be transferred to the conical flask (and the flask capped) at a place
well away from the flame. Since the vapours from propanone are very irritating the
laboratory should be well ventilated. Propanone is a severe eye irritant and will degrease
the skin.
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Disposal of wastes
Flush to the foul water drain with large quantities of water.
Specimen Results (Method 1)
Mass of flask, cap and rubber band
Mass of condensed vapour, flask, cap and
rubber band
Mass of condensed vapour
Atmospheric pressure
760 mmHg = 101325 Pa
Atmospheric pressure
Temperature of boiling water
Temperature of boiling water
Gas Constant, R = 8.314 J K-1 mol-1
115.15 g
115.67 g
0.52 g
756 mmHg
100792 Pa
100 0C
373 K
284 cm3
2.84 x 10-4 m3
Sample calculations
PV = nRT
 n = PV / RT
 n = 100792 x 2.84 x 10-4 / 8.314 x 373
 n = 9.23 x 10-3 moles
n = m / Mr
 Mr = m / n
 Mr = 0.52 / 9.23 x 10-3
 Mr = 56.33
Specimen Results (Method 2)
Mass of syringe, needle and contents
before injection
Mass of syringe, needle and contents after
injection
Mass of vapour
Atmospheric Pressure
15.39 g
15.27 g
0.12 g
756 mmHg
10
760 mm Hg = 101325 Pa
Atmospheric Pressure
Temperature inside heater
Temperature of vapour
Volume of heated air
Volume of heated air and vapour
Volume of vapour
Volume of vapour
Gas Constant, R = 8.314 J K-1 mol-1
100792 Pa
98 0C
371 K
7 cm3
79 cm3
72 cm3
7.2 x 10-5 m3
Sample calculations
PV = nRT
 n = PV / RT
 n = 100792 x 7.2 x 10-5 / 8.314 x 371
 n = 2.35 x 10-3 moles
n = m / Mr
 Mr = m / n
 Mr = 0.12 / 2.35 x 10-3
 Mr = 51.1
1. Why is it necessary that the liquid used in this experiment is volatile?
The liquid must be capable of forming a vapour at the temperature of the experiment so
that the ideal gas equation can be applied. The equation applies with greatest accuracy to
those vapours that are most like ideal gases. Gases are least ideal when on the point of
condensing, so the greater the difference between the boiling point of the liquid and the
temperature of the reaction the more accurate the results will be.
2. Name another technique for measuring relative molecular mass.
Mass spectrometry
3. Since the vapour is not an ideal gas, which quantity measured in the experiment is
most likely to introduce inaccuracy in the result and why?
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The temperature and pressure of the surroundings were measured. These measurements
are independent of whether the gas is ideal or not. The gas constant is given. The
measurement of volume is the measurement that would be expected to be least accurate
when used in the ideal gas equation, when compared to the volume that would be
obtained if an ideal gas could be used. A real gas occupies a smaller volume than an ideal
gas because of intermolecular attractions in real gases, e.g. hydrogen bonding, van der
Waals’ forces etc.
4. If a small drop of water were present in the flask used in Method 1 or the gas syringe
used in Method 2, how would this affect the results?
The small drop of water would vaporise during the experiment and occupy quite a large
volume. The reading for the volume of the volatile liquid's vapour would be far too large
and the result calculated for Mr very inaccurate as a result. It would be too small.
5. From your results calculate the density of the vapour of the volatile liquid at the
temperature of boiling water.
Density = mass / volume. For example, using the data in the sample results:
Method 1: density = 0.52/284 = 0.0018 g/cm3
Method 2: density = 0.12/72 = 0.0017 g/cm3
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