MHS Chemistry
Molar Volume of a Gas

The purpose of this lab is to determine the molar volume of hydrogen gas by applying what knowledge of pressure and moles to an actual reaction.  Recall that "molar volume" means the volume occupied by one mole of something.  In this case, the "something" will be a gas under ideal conditions.


  1. Assemble the apparatus a shown by your instructor.
  2. Obtain about 5 cm of magnesium ribbon.  Measure the mass exactly.
  3. Wrap the magnesium ribbon in a cage of copper wire.  Leave a few inches of copper hanging free for attachment.
  4. Place 15 to 20 mL of  3.0 M HCl in the gas collection tube.  Gently top off the tube with distilled water.
  5. Place the magnesium ribbon in the tube, hooking the copper wire from the cage over the lip of the gas collection tube.  Wedge the whole thing together with a small two-hole stopper.  Water should spill out of the cork holes.
  6. Cover the holes with your thumb, and invert the collection tube into the large graduated cylinder of water.  Clamp it in place, leaving room between the tube and the bottom of the water.  Observe the acid flowing down inside the tube, and the formation of gas bubbles.  Make sure that ALL of the magnesium ribbon reacts.  If some of it floats up and sticks to the wall, slosh the tube around to keep the Mg submerged.
  7. If some of the magnesium remains unreacted, repeat steps 1-6 with a little less ribbon or a little more acid.
  8. Raise or lower the tube so that the water level inside and outside are the same.  Record the exact volume of gas collected.
  9. Record the room temperature (°C) and pressure (mmHg) from the back of the room.  Clean up your lab station, and put your goggles away.
  1. Use the mass of magnesium to calculate the moles of magnesium used.
  2. Determine the pressure from water vapor in the hydrogen collected (the gas is called “wet hydrogen” because it contains water vapor).  This pressure is only dependent on temperature.  With temperature in °C, the pressure of water vapor, in mmHg, is given as

  3. P = 0.0334T2 – 0.243T + 8.9935
  4. Use the total pressure and the pressure from water vapor to determine the partial pressure of hydrogen gas (this is the pressure from just the hydrogen).   Use the equation:   P(total) = P(water) + P(hydrogen)
  5. Use the relation P1V1 = P2V2 to determine the volume of pure (dry) hydrogen gas at 760 mmHg.
  6. Write the balanced equation for the reaction of magnesium metal with hydrogen chloride.
  7. Use the moles of magnesium you started with to determine the moles of hydrogen gas produced.
  8. Determine the volume of one mole of dry hydrogen gas at room temperature.  Give this volume in mL/mol and L/mol.
2. mass of Mg ribbon used __________ ___
8. measured gas volume __________ ___
9. room temperature __________ ___
room pressure __________ mmHg







a. Moles of magnesium in ribbon __________ ___
b. Vapor pressure of water __________ ___
c. Pressure of dry hydrogen __________ ___
d. Volume of dry hydrogen __________ ___
e. Balanced reaction


f. moles of hydrogen gas produced __________ ___
g. molar volume of hydrogen gas @ room T __________ mL/mol
__________ L/mol

Shows all calculations in the space below.
Each person should turn in their own lab.

[Molar Volume of a Gas score sheet][MHS Chem page]