Name: ______________________
Period: ______________________
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.
In this experiment, you will react a known mass of magnesium with excess
hydrochloric acid according to the reaction:
Mg(s) + 2HCl(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 produced and
the volume occupied by the gas. The number of moles of hydrogen will be determined
using the balanced equation shown above. By determining the mass of magnesium that
reacts and the number of moles that this mass is equal to, you can determine the number
of moles of hydrogen gas that are produced. The volume of the hydrogen gas produced
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, measured under laboratory conditions, to
the volume the sample of gas would occupy at STP. The collected data(number of moles
and volume at STP) 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.
EQUIPMENT AND MATERIALS
Gas measuring tube
One-hole stopper (for gas tube)
Ring stand
Utility clamp
thermometer
cotton thread
beaker
bucket
10mL graduated cylinder
metric ruler
safety glasses
Magnesium ribbon (Mg)
3M hydrochloric acid (HCl)
PROCEDURE:
1. Obtain a piece of Mg ribbon. Measure the length of the piece of ribbon to 0.1cm.
Record the length as (a) in the data table. Also record the mass of 1 meter of Mg
ribbon. This data will be provided to you. Record as (b).
2. Coil the piece of Mg so that it will fit easily into the gas-measuring tube. Obtain a
piece of cotton thread about 15cm long. Tie one end through the coils of the piece of
Mg, leaving about 10cm of thread free.
3. Carefully measure out 10.0mL of 3M HCl. Pour the HCl into the gas-measuring tube.
4. Tilt the gas-measuring tube slightly. Using a beaker, slowly fill the gas-measuring
tube with water at room temperature. Try to avoid mixing the acid and water.
5. Lower the piece of Mg ribbon 4 or 5 cm into the gas-mesuring tube. Drape the thread
over the edge of the tube and insert the one-hole stopper into the tube.
6. Fill a bucket ¾-full of water.
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 bucket. Hole the tube in place so
that the stoppered end is a few centimeters above the bottom of the bucket. Record
your visual observations as (g) in the data table.
8. Let the apparatus stand for about five minutes after the Mg has completely reacted.
Then, tap the sides of the gas-measuring tube to dislodge any gas bubbles that may
have become attached to the side of the tube.
9. Move the tube up or down (to equalize pressure) until the water level in the tube is
the same as that in the bucket. On the scale of the gas-measuring tube, read the
volume of the gases in the tube. Record this volume as (c) in the data table.
10. In the data table, record the room temperature (d) and the barometric pressure (e).
11. Repeat the experiment.
OBSERVATIONS AND DATA: (USE UNITS!!)
Trial 1
a.
Length of Mg
ribbon
b.
Mass of 1m of Mg
ribbon
c.
Volume of H2 gas
in tube
d.
Room temperature
e.
Barometric pressure
f.
Water vapor pressure
at room temperature
(chart posted in lab)
g.
Visual observations
Trial 2
Average
CALCULATIONS: (SHOW WORK, USE UNITS AND SIG DIGS!)
1. Find the mass of Mg strip using the mass of 1.00m of Mg as a conversion factor.
2. Calculate the number of moles H2 gas produced (which is equal to the number of
moles of Mg reacted: use values from #1 above)
3. Find the pressure exerted by the H2 gas in the tube (PH2 = Pbarometric – PH2O)
4. Convert room temperature from oC to K.
5. Find the volume of the H2 gas at STP.
P1V1 = P2V2
T1
T2
Where:
P1 = PH2 (from calculation 3)
V1 = experimental value of H2 (c)
T1 = room temperature (in K)
T2 = 273 K
P2 = 760 mm Hg
V2 = volume of H2 at STP
6. Find the volume of 1 mole of H2 gas at STP.
Moles of H2 gas (from calculation 2) = 1 mole
Volume of H2 gas (V2)
x mL
x = _______________mL/mol = _______________________L/mol
CONCLUSIONS AND QUESTIONS: (SHOW ALL WORK!)
1. The accepted value for the molar volume of a gas is 22.4L. How does your
experimentally determined value compare with this accepted value? Calculate your
percentage error.
2. List 3 possible sources of error in this experiment.
3. Find the volume of the following masses of gases at STP:
a. 80.0g O2:
b. 10.0g H2:
b. 14.0g N2:
d. 66.0g CO2:
4. How many liters would the following number of moles of any gas occupy?
a. 0.25 mol
b. 0.50 mol
c. 1.0 mol
d. 2.0 mol
e. 2.5 mol
5. What happens to the other product of the reaction used in this experiment?