Molar Volume Lab – Chemistry Block: _____
Name: _____________ Lab Partner: ______________
In this lab, you will be generating hydrogen gas by the chemical reaction
Mg (s) + 2 HCl (aq)  MgCl2 (aq) + H2 (g)
The reaction will be carried out with excess HCl, hence the volume of hydrogen gas produced is
dependent on the amount of magnesium used as well as the pressure and temperature at which the
reaction is run.
Each group will be given a volume of hydrogen gas to produce and a portion of this lab’s grade depends
on you achieving this goal. With the data from this lab, you will additionally determine the molar
volume of hydrogen at STP. Record everything in your lab notebook.
Your target volume of H2 is: ___________ mL
Today’s barometric pressure is: _________ mm Hg
Read the procedure and make the appropriate data tables in your lab book before you come to class.
Procedure:
1. Record your target volume of H2 gas and the barometric pressure in your lab notebook.
2. Fill a 400 mL beaker ¾ full with tap water.
3. Figure out the grams of Mg needed to produce the required amount of hydrogen gas. The
Mg ribbon has a linear density of ______ g/cm. Obtain the required Mg by
cutting the Mg ribbon with a scissors. Then weigh and record the mass of
your Mg.
4. Obtain 10 cm piece of copper wire and make a cage to hold the Mg ribbon
(see the examples). Firmly place the Mg ribbon in the cage.
5. Insert the straight end of the copper wire into a one-hole rubber stopper. The
cage should be about 5 cm from the stopper. The copper wire and cage
should not move, so you may need to bend it a bit.
6. Obtain 15 mL of HCl (hydrochloric acid) in a 25 mL graduated cylinder. Be
careful you are working with a strong acid.
7. Using a ring stand and a clamp, clamp your eudiometer tube with the open end up and using
a funnel, carefully pour the acid into the tube.
8. Using a wash bottle, slowly fill the tube with distilled water. Work very slowly to void mixing
the acid and water layers. Fill the tube to the top so that no air remains in the tube.
9. Insert the copper wire/cage/stopper in the eudiometer tube.
10. Place your finger over the hole in the rubber stopper and invert the tube and carefully lower
the stoppered end into the 400 mL beaker filled with tap water. Clamp the tube in place.
Hopefully, there are no air bubbles. Your apparatus should look like the following:
11. Watch what happens. Record your observations.
12. After the Mg has reacted, let the tube stand for 5 minutes. Record the volume of the gas.
We’ll call this Volume 1.
13. Obtain one of the 500 mL or 1 L graduated cylinders in the lab that
have been filled with water.
14. Cover the whole of the stopper with your finger and transfer the
eudiometer tube to the graduated cylinder. You do not want any
water/acid to leave the tube.
15. Gently move the eudiometer tube up and down in the cylinder
until the water level inside the tube is the same as the water level
in the graduated cylinder. This is done to equalize the pressure in
the tube with the pressure outside of the tube. Record the
volume of the gas. We’ll call this Volume 2. Record the
temperature of the water.
16. Empty out the eudiometer tube in the sink and bring your copper
cage, rubber stopper, and eudiometer tube back to the demo desk.
Analysis and questions:
1. Compute the theoretical number of moles of H2 gas produced from the reaction and the amount of
Mg used.
2. Using the class data table of Vapor Pressure of H2O at Different Temperatures, calculate the partial
pressure of the H2 gas.
3. Use the combined gas law to covert Volume 2 to the ideal volume the H2 gas would occupy at STP.
(Use the partial pressure of H2 computed in #2).
4. Find the molar volume of H2 at STP by dividing the ideal volume computed in #3 by the theoretical
moles of H2 gas found in #1.
5. Compute the percent error (1 mole of gas occupies 22.4 L at STP).