Determination of the
Molar Volume of Hydrogen Gas
Purpose

To determine the volume of one mole of
hydrogen gas at standard conditions.
Concept

Molar Volume is the volume occupied by
one mole of a gas at standard conditions.
(Theoretically 22.4 liters per mole.)
– The student should describe an application of this concept.
Safety
Name
H
F
R
Special
Mg Ribbon
2
3
2
Flammable metal
6M HCl
3
0
3
Extreme Corrosive
Procedure

Use iron wool to polish approximately 5 cm
of magnesium ribbon. Then accurately mass
the sample of magnesium.
Procedure.2

With a 20 cm piece of thread, tie one end
around the middle of the Mg ribbon and
bend the Mg ribbon into a zig-zag pattern.
Procedure.3

Prepare a 600ml beaker
two thirds full with tap
water. The eudiometer
tube assembly will be
placed in this beaker
later in the experiment.
Procedure.4

Prepare the eudiometer tube by placing 15
ml of 6M Hydrochloric acid (HCl) in the
tube, then fill the tube to the top with
distilled water, being careful to minimize
the mixing of the acid.
Procedure.5

Quickly place the Mg
ribbon in the top of the
eudiometer tube and
pinch the thread between
the stopper and the wall
of the eudiometer tube.
Procedure.6

With out letting in
any air bubbles,
invert the eudiometer
tube assembly into
the water bath;
allowing the contents
the ability to flow
into the water bath.
Procedure.7
Make and record observations.
Notice the density
currents caused by
the ‘sinking’ HCl.
It makes the
solution look oily.
Once the acid reaches
the Mg it begins to
produce H2 gas.
Procedure.8
Record the volume
of the gas produced.
44.61 ml
(read at apex of meniscus)
Procedure.9
Record the height of the
water column, in
millimeters, above the
level of the water bath.
11.65cm high or
116.5 mm of water
Procedure.10
Record the temperature and the
atmospheric pressure.
Calculations.1

The pressure of the dry H2 gas must be
calculated.
 There are two corrections to be made.
– The water vapor mixed in with the collected H2
must be corrected.
– The pressure difference between the inside of
the tube and outside the tube must be accounted
for.
Calculations.2

The pressure in the tube is less than the
pressure outside the tube
 The difference in pressure can be calculated
by dividing the difference in water levels
expressed in mm of H2O by 13.6 because
mercury is 13.6 times more dense than
water.
116.6 mmH2O
1.00 mm Hg
13.6 mmH2O
= 8.57 mmHg
Calculation.3

The total pressure inside the tube is the
atmospheric pressure minus the water
column pressure difference.
774.2 mmHg – 8.57 mmHg = 765.6 mmHg
Calculations.4

Because there is H2 and H2O in the tube, the
pressure of the water vapor must be
subtracted.
 Look up the water vapor pressure for the
temperature of the gas.
 The vapor pressure of water at 22.7°C is
20.7 mmHg
765.6 mmHg - 20.7 mmHg = 744.9 mmHg
Conclusion

A good conclusion includes
– a summary
– results
– sources of error
– describes an error propagation.