LOHS General Chemistry
Lab 21 Molar Volume of a Gas
Introduction
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 condition of temperature and pressure.
A special name is given to the volume occupied by 1-mole samples of gases at STP. This is called the molar
volume.
The basis of this experiment is the following reaction, in which you will react a known mass of magnesium with
excess hydrochloric acid in a single replacement reaction. You may write the reaction below:
The gas product is what is of most interest in this experiment (include physical states in the reaction). 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 gas produced is 1:1.
Therefore, by determining the mass of magnesium that reacts and the number of moles that this mass is equal
to, you will also determine the number of moles of hydrogen gas produced. The volume of hydrogen gas
produced will be measured directly on the scaled of the graduated cylinder. The gas laws of Boyle and Charles
will be used to correct this volume, measured under laboratory condition, to the volume the sample of gas
would occupy at STP. The collected data (number of moles at STP) will be used to calculate the molar volume
of the hydrogen gas.
Purpose
Determine the volume of 1 mole of hydrogen gas at STP using experimental data, known mathematical
relationships and a balanced chemical equation.
Equipment
graduated cylinder, 10 mL
one hole stopper (00)
thermometer
beaker, 600 mL
2 beaker, 50 mL
metric ruler
micropipet
lab apron
safety goggles
Materials
magnesium ribbon (Mg)
3 M hydrochloric acid (HCl)
copper wire
Procedure
1) Cut two pieces of magnesium between 4-5.5 mm. Measure the length of the piece of ribbon (+/- 0.1 cm).
Record the length in your data table. Also record the mass of 1 meter of magnesium ribbon.
2) Cut a piece of copper wire approximately 20 cm.
3) Wrap the piece of copper wire around one of the pieces of magnesium; as if you wanted it stay warm and
cozy in the caress of the copper.
4) Obtain 3 mL of 3M hydrochloric acid (HCl). Handle the acid with care. Pour a SMALL amount of acid
into one of your 50 mL beakers.
5) Fill the 600 mL beaker 4/5 up with water.
6) Add the 3M hydrochloric acid to the 10 mL graduated cylinder.
7) Fill the remaining 50 mL beaker with water and use the micropipet to gently add water to the graduated
cylinder until it is full (to try to eliminate any excess gas from being trapped).
8) Submerge the wrapped piece of magnesium. A few cm above the 10 mL mark on the graduated cylinder.
Flatten the excess copper wire down the side of the graduated cylinder. Place the 1-hole stopper in to the
graduated cylinder.
9) Place your finger over the hole in the rubber stopper and invert the graduated cylinder. Lower the stoppered
end of the cylinder in the beaker of water. Once the stopper is submerged allow the cylinder to rest at a
diagonal in the bottom of the beaker.
10) Allow the magnesium to react completely. Wait 5 minutes after the magnesium is finished reacting. Tap
the sides of the cylinder to dislodge any gas bubbles that may have become attached to the sides of the
cylinder.
11) Move the cylinder up and down (to equalize pressure) until the water level in the tube is the same as that in
the beaker. On the scale of the cylinder, read the volume of the gas produced. Record the volume in the
data table. Remember that the cylinder is upside down.
12) Record the room temperature and temperature of the water.
13) Remove the graduated cylinder and rinse it under running water, be careful as some hydrochloric acid may
still be present.
14) Repeat the procedure and conduct a second trial.
15) After finishing your second trial vigorously rinse your beaker and graduated cylinder.
Observations and Data
Trial 1
a. length of Mg ribbon
b. mass of 1 meter of Mg
c. volume of H2 gas in tube
d. room temperature
e. barometric pressure
f. temperature of water
g. water vapor pressure
h. visual observations:
Trial 2
Average of 2 trials
Calculations
1) Find the mass of the magnesium ribbon:
2) Calculate the number of moles of magnesium and hydrogen gas reacted: (use your stoichiometry)
3) Find the pressure exerted by the H2 gas in the tube: (Hint: PH2 = Pbarometric - PH2O)
4) Convert temperatures to Kelvin:
5) Find the volume of H2 gas at STP: (Hint: Use the combined gas law, P2, T2, V2 are at STP)
6) Find the volume of 1 mole of H2 gas at STP:
Moles of H2 gas
Volume of H2 gas (V2)
x=
=
mL/mole =
1 mole
x mL
L/mole
Conclusions and Questions
1) The accepted value for the molar volume of a gas is 22.4 L. How does your experimentally determined
value compare with this accepted value? Calculate your percent error.
2) What are some possible sources of error in this experiment?
3) Find the volume of the following masses of gases at STP:
a. 80 g O2
b.10 g H2
c. 14 g N2
d. 66 g CO2
4) Find the volume of the following number of moles that any gas occupies at STP.
a. 0.25 mole
b. 0.5 mole
c. 1 mole
d. 2 moles
5) What happens to the other product of the reaction used in this experiment?
e. 2.5 moles