Molar Volume PRELAB - verifying Avogadro's 22.4 L hypothesis
In this lab, a sample of hydrogen gas will be produced via a reaction of magnesium metal
with hydrochloric acid. This gas will be collected via a water-displacement method.
The purpose of collecting the hydrogen gas will be to verify Avogadro's molar
volume hypothesis -- that 1 mole of any gas will occupy a volume of 22.4 liters at STP
conditions. We will not carry out the lab at STP conditions, of course; but we can use the
Combined Gas Law to convert our results obtained at laboratory conditions, to STP
conditions.
When a gas is collected over water, a small amount of water will evaporate into the
collected sample, resulting in a small water vapor pressure contribution to the total
measured pressure. The pressure of the collected sample is the sum of the partial
pressures of the gas sample and the water vapor. The pressure of the gas sample only
can be calculated by subtracting the partial pressure of the water vapor from the total
pressure recorded. This is an application of Dalton's law of partial pressures.
Procedure:
1. Read through the procedure, and write down 2 appropriate safety precautions for this
lab. Write them under a heading of Safety before the Procedure section.
2. Record the barometric pressure of the lab, as directed by your instructor.
3. Obtain a piece of magnesium ribbon, about 3 cm long. Using a metric ruler, measure
its length to the nearest 0.01 cm, and record in your data table.
4. Record in your data table, the mass of the 1.00 meter length of magnesium ribbon that
your sample was cut from.
5. Carefully fold up the piece of magnesium in an "s" shape. Do this carefully! The
magnesium is brittle.
6. Using the piece of copper wire attached to the stopper, form a cage around the piece of
folded-up magnesium. See the picture below.
7. Carefully pour 10.0 mL of 6 M hydrochloric acid into the
empty eudiometer (gas collection tube).
8. Inclining the eudiometer, carefully add tap water on top
of the acid in the tube, by pouring tap water from a
250 mL beaker down the inside of the eudiometer tube.
You don't want to disturb the acid at the bottom of the
tube...you are just placing a layer of water over the
more-dense acid. Completely fill the tube with water let it overflow slightly to make sure it is completely filled.
9. Insert the copper wire/stopper into the eudiometer, so that water overflows slightly. You
want to make sure that no air is trapped inside the tube.
10. With a finger over the hole in the stopper, quickly invert the eudiometer into the large
jar filled with water. Both sides of the lab table will use the same large water jar.
Clamp the tube to a ring stand, so that the stopper is near the bottom of the large
water jar. See picture below.
11. Whoever just dipped their hand into the large water jar, should wash their hands off.
12. Watch the reaction occur. Take notes on the reaction.
13. After the reaction has ended, wait 5 minutes, and then take the temperature of the
water bath. We will assume that this temperature will equal the temperature of the gas
produced. Record this temperature.
14. Loosen the clamp, and adjust the eudiometer so that the level of the water inside the
tube, equals the level of the water outside the tube. Then re-clamp. By doing this, we
are making sure that the pressure of the gas sample we produced, equals the
barometric pressure of the lab. Make a note in your data table, indicating that the
pressure of the gas sample equals the barometric pressure of the room.
15. Record the volume of gas collected, by carefully reading the eudiometer tube
markings. Don't move the eudiometer while doing this! It should be clamped so that
the level of the water inside the tube, equals the level of the water outside the tube.
16. The gas sample you collected, is really a mixture of hydrogen gas and water vapor.
Using Dalton's law of partial pressures, and the table of water vapor pressures at
various temperatures from a class worksheet, record in your data table the partial
pressure of the water vapor trapped in the tube.
17. Remove the eudiometer, and un-coil the copper wire cage. Wash your hands.