LAB 9:
DETERMINATION OF THE MOLAR VOLUME OF A GAS
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This lab does NOT require a lab report.
This lab will be completed as a class demonstration, but you will be responsible
for working up your own data.
I Introduction
As can be easily calculated from the Ideal Gas Law, one mole of gas at STP (1 atm
pressure and 0oC) will occupy a volume of 22.4 L. This number, called the “molar
volume of a gas”, is a convenient standard for measuring amounts of gases and is
worth remembering. In this lab, we will calculate the molar volume of hydrogen gas.
Since we are assuming that the identity of a gas does not affect its physical properties
(including volume), then this would be the molar volume of ALL gases, as well.
Gases are generally difficult to work with: they are difficult to contain, difficult to
measure, and difficult to prevent them from interacting with their surroundings (such
as dissolving in water or reacting with metals). In this lab, we will use the classic gas
collection method of “water displacement”: we produce an insoluble gas (in this case,
hydrogen) and allow it to collect in a container of water which is open at the bottom
to a reservoir of water. Being less dense than water, the hydrogen gas floats to the
top and forces (“displaces”) the water down and out of the container. The container
used in this lab is one specifically designed to measure the volume of a gas and is
known as a eudiometer tube.
Water Displacement Method
of Gas Collection
Now that the purpose and equipment of the lab have been discussed, all that remains
is the gas itself. Hydrogen gas is one of the easiest gases to produce in the lab and has
the added advantage of being virtually insoluble in water (unlike carbon dioxide and
ammonia gases, which are also easy to produce). In this experiment, you will produce
hydrogen by reacting a strip of magnesium ribbon with hydrochloric acid, as shown in
the following unbalanced equation:
Mg(s) + HCl(aq)  H2(g) + MgCl2(aq)
Lastly, the molar volume of a gas is typically measured at STP. Needless to say, we
won’t be cooling the lab down to 273 K. Instead, we will use room temperature and
pressure and the combined gas law (shown below) to convert your measured molar
volume into the standard molar volume as it would exist at STP:
P1V1
P2V2
=
T1
T2
Equipment and Reagents
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Eudiometer tube
Analytical balance
Thermometer
Access to a barometer
Large reservoir of tap water
Large cylinder of room temperature tap
water
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Magnesium ribbon
6 M HCl (hydrochloric
acid)
Distilled water
Tap water
Cautions
 The hydrochloric acid is 6 Molar: treat with care! Wear your goggles and
aprons!
Procedure
1. Prepare a large reservoir of water, if one is not already available.
2. Cut off a piece of magnesium ribbon that’s 4-5 cm long and weigh and record its
mass.
3. With the eudiometer tube upside down, add 10 mL of 6 M HCl from the buret of
acid. This allows you to calibrate your tube, as well: do you need to adjust your
volumes any? Fill the rest of the tube with distilled water, using a wash bottle
for the last few milliliters. You want the tube FILLED TO THE BRIM!
4. Drop in the strip of magnesium, let it fall until it starts bubbling, and then,
WITHOUT LETTING IN ANY AIR BUBBLES, invert the tube and submerge its end
underwater in your reservoir.
5. The magnesium should remain floating as it produces hydrogen gas. The
reaction should be vigorous and quick. Often the magnesium becomes stuck to
the side of the tube; gently swirl the tube if it does, not letting the end of the
tube break the surface of the water.
6. Once the reaction has stopped, plug the tube with your finger and transfer it to
the large cylinder of water. Submerge the tube until the water level in the tube
is the same as that in the cylinder. This equalizes the pressures in the tube and
cylinder, and allows you to indirectly measure the pressure of hydrogen by
simply measuring the barometric pressure of the room.
7. Record the volume of the gas, the temperature of the water, and the
barometric pressure outside.
8. Clean up by letting your gas escape (sorry: no fires (but maybe a dog bark or
two)) and draining your tube and cylinder.
Data Analysis
Include the following in your lab Data Sheet:
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Lab procedure and observations
Part 1: Determination of Your Molar Volume at Lab Conditions
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Balanced equation
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Mass and mols of magnesium used
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Stoichiometry and mols of H2 produced
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Volume of gas collected
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Volume of 1 mol of gas at lab conditions (using Avogadro’s Law and/or simple
proportions)
Part 2: Standardization and Comparison with “Real” Molar Volume
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Temperature of water
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Barometric pressure outside
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Partial pressure of water at given temperature (see reference literature)
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Corrected pressure (barometric pressure minus water partial pressure)
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Corrected volume of gas at STP (using combined gas law. The result is your
molar volume at STP)
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Percent error of results: Your molar volume compared to 22.4 L
Questions to Answer
1. What were some sources of error in your lab? What would you do differently if
you could repeat the lab?
2. Suppose that a bubble of hydrogen gas slipped out of the tube and your
measured volume was less than it should’ve been. Would this error make your
calculated molar volume too big, too small, or have no effect? Explain.
3. Magnesium corrodes easily by reacting with the air. If your magnesium was not
bright and shiny when you began, perhaps you too, were a victim of corrosion.
Suppose that your magnesium had corroded, such that your initial piece of
magnesium contained less Mg than it really did. Would this error make your
calculated molar volume too big, too small, or have no effect? Explain.