Molar Volume of a Gas
(Create a data table for all data
needing collection and recording
during lab. Use straight edge and units
to be recorded as well)
Purpose: To experimentally determine the molar volume of a gas
Procedure:
1.
2.
3.
4.
5.
6.
7.
Record the mass of the piece of magnesium provided.
Record the atmospheric pressure that is written on the board.
Fold the magnesium ribbon through the end of the copper wire in the small rubber stopper so that the magnesium
ribbon will remain in place during the experiment.
Fill the large graduated cylinder to the top with tap water and place it in the plastic or metal trough. The trough
will catch any water that overflows the cylinder as the reaction proceeds.
Carefully pour about 15 mL of 3M HCl into the gas collecting tube, using the markings on the tube as a guide.
(You do not need to measure the HCl with a graduated cylinder.)
Angle the gas collecting tube and slowly fill the rest of the tube with tap water by pouring water from a beaker into
the tube. Fill it to the very top. Then insert the rubber stopper with the copper wire and the magnesium. Some
water should come out when you insert the stopper if you have filled the tube correctly.
Holding your finger over the opening in the rubber stopper, invert the tube into the large graduated cylinder.
Remove your finger only after the opening of the tube is below the water level. Slowly lower the tube into the
graduated cylinder. Observe the reaction as it takes place.
Mg + 2HCl → H2 + MgCl2
8. When no more gas is being produced, gently tap the tube to allow all the gas to rise. Then let the tube sit
undisturbed for 3-4 minutes to allow the temperature to stabilize. Record the temperature of the water in the
graduated cylinder.
9. Carefully raise the tube so that the level of water in the tube is the same as the level of water in the graduated
cylinder. This assures that the total pressure of the gas in the tube is the same as the atmospheric pressure. Read the
volume of the gas in the tube using the graduations on the side of the tube. Record this volume in your data
table.
10. Let me know if you would like for me to ignite your hydrogen gas. Clean up your work area.
Calculations:
1.
2.
3.
4.
5.
6.
Calculate the number of moles of magnesium in your sample.
Look up in your textbook or a Handbook of Chemistry and Physics the value for the vapor pressure of water at
the temperature you recorded for your experiment (in step 8). Subtract this value from the atmospheric pressure
to obtain the value for the pressure of the hydrogen gas alone.
Using the volume of gas you measured in your collecting tube and the pressure you calculated in step 2 above,
calculate the volume you would have collected at STP.
Use the balanced equation above to calculate how many moles of hydrogen should be produced (theoretical
yield) from the number of moles of magnesium in your sample.
Using the STP volume you calculated in step 3 as the volume occupied by the number of moles of H2 you
collected, calculate the volume of H2 you would have produced if you had started with 1.0 mol of magnesium.
Calculate your percent error, assuming that the theoretical value is 22.4 L.
Questions:
1.
2.
3.
4.
5.
How would the volume of hydrogen collected differ if you had used only half the mass of magnesium you started
with?
How would changing the concentration of HCl used affect the results of this experiment?
How does Avogadro’s hypothesis relate to the results of this experiment?
Using what you have learned in this experiment, explain why a balloon filled with helium will float up, but an
identical balloon filled to the same point with air at the same temperature and pressure will not float up.
If instead of producing hydrogen gas, you used a reaction that produced carbon dioxide gas, how would the molar
volume you determined differ?