Experiment 7B - Molar Volume of a Gas
PRE-LAB DISCUSSION
Avogadro’s hypothesis states that equal volumes of all gases contain
equal numbers of molecules under the same conditions of temperature and
pressure. It follows from this hypothesis that all gas samples containing
the same number of molecules will occupy the same volume under the
same conditions of temperature and pressure. A special name is given to
the volume occupied by 1 mole samples of gases at STP. This volume is
called the molar volume. In this experiment, you will make an experimental
determination of the molar volume of hydrogen gas.
The basis of this experiment is the following reaction in which you will
react a known mass of magnesium with an excess of hydrochloric acid to
produce the substances shown:
Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)
The hydrogen gas is the product that is of interest to you in this
experiment. You will make an experimental determination of the number of
moles of hydrogen molecules produced and the volume occupied by these
molecules. The number of moles of hydrogen will be determined indirectly.
The balanced equation for this reaction shows that the molar ratio of
magnesium reacted to hydrogen produced is 1:1. Therefore, by
determining the mass of magnesium that reacts and using a mass to mole
stoichiometric calculation, you will be able to determine the number of
moles of hydrogen gas produced. The volume of hydrogen gas will be
measured directly on the scale of a gas-measuring tube. The gas laws of
Boyle and Charles will be used to correct this volume of hydrogen since it
is being collected under laboratory conditions instead of STP. Your
corrected volume and your calculated number of moles of hydrogen will be
used to calculate the molar volume of the hydrogen gas.
PURPOSE
To determine the volume of 1 mole of hydrogen gas at STP using
experimental data, known mathematical relationships, and a balanced
chemical equation.
MATERIALS
gas-measuring tube
barometer (class)
ring stand
utility clamp
thermometer (class)
magnesium ribbon
safety goggles
6 M HCl
400 mL beaker (filled to 300 mL mark with
distilled water)
one-hole stopper (fitted for gas tube)
2000 mL graduated cylinder (filled with tap water)
10 mL graduated cylinder
blue food coloring
cotton thread
PROCEDURE
1) Obtain a piece of magnesium ribbon from you teacher (approx. 5
cm). Find the mass of the ribbon and record in the data table.
2) Obtain a piece of thread about 15 cm long. Tie
one end of the thread around the piece of
magnesium ribbon, leaving about 10 cm of
thread free. Bend the ribbon into a little ball so
that it will easily fit into the gas-measuring tube
as shown in figure 1.
3) CAUTION: HANDLE ACIDS WITH CARE! Obtain about 10mL of 6M
HCl using the 10mL graduated cylinder. Add 3 drops of blue food
colorings to the acid. Carefully pour the HCl into the gas-measuring
tube.
4) Tilt the gas-measuring tube slightly. Using the 400 mL beaker of
water, slowly fill the gas-measuring tube with water. Try to avoid
mixing the acid and water as much as possible – the blue dyed acid
should not be greatly disturbed.
5) Lower the piece of magnesium ribbon 4 or 5 cm
into the gas-measuring tube. Drape the thread
over the edge of the tube and insert the one-hole
stopper into the tube to hold the thread in place as
shown in figure 2.
6) Place the 400 mL beaker of water in the position
shown in figure 3.
7) Place your finger over the hole in the rubber
stopper and invert the gas-measuring tube.
Lower the stoppered end of the tube into the
400 mL beaker of water. Clamp the tube in
place so that the stoppered end is a few cm
above the bottom of the beaker as shown in
figure 3. Record your visual observations
under the date table.
8) Let the apparatus stand about five minutes
after the magnesium has completely reacted.
Then tap the sides of the tube to dislodge any
gas bubbles that may have become attached to
the sides of the tube. Place your finger over
the hole in the stopper and transfer the tube to
the 2000 mL graduated cylinder of water.
Lower the end the tube in the water and remove
you finger from the hole.
9) Move the tube up or down (to equalize
pressure) until the water level in the tube is at
the same height as the water level in the 2000
mL graduated cylinder. On the scale of the
gas-measuring tube, read the volume of the gas
in the tube. Record this volume in the data
table.
10) In the data table, record the room temperature
and barometric pressure.
CHEMICAL EQUATION
Mg(s) + 2 HCl(aq)  MgCl2(aq) + H2(g)
DATA
Mass of Mg ribbon before reaction
Volume of hydrogen gas
Room temperature
Room barometric pressure
Observations of reaction:
CALCULATIONS
1) Calculate the number of moles of Mg used.
2) Calculate the number of moles of H2 produced (hint: mole ratio)?
3) Calculate the molar volume of H2.
Vnew =
Vold X
moles (new)
moles (old)
Example: If at a certain temp. and pressure 0.00174 mol of H2 occupy 40.0
mL, then the molar volume would be:
Vnew =
0.0400 L X
1.00 mol
0.00174 mol
= 23.0 L
4) The accepted value for the molar volume of a gas at STP is 22.4 L. Find
your percent error.
% error =
Vexpt - Vaccepted x 100%
Vaccepted
QUESTIONS
1) For the following possible errors, give the direction they would cause
your results to deviate from the accepted value of 22.4 L/mol at STP.
Explain your answers.
a) air bubbles were introduced when tube is turned over in step 7
b) not all the Mg ribbon reacted
c) not all the MgO coating is removed from the Mg ribbon before
beginning the experiment
d) you read the volume when the water level in the tube was higher
than outside the tube in step 9
2) Find the volume of 80 g O2 at STP.
3) How many liters would 0.25 mol of any gas occupy at STP?