Molar Volume of Hydrogen


Background Information

The term, molar volume, refers to the volume of one mole. Since volume varies with temperature and pressure, it would be meaningless to compare volumes measured under different conditions. It is therefore customary to make the measurements under convenient laboratory conditions and then convert the measurements to the volume at standard temperature and pressure (STP, 0°C and 1 atm).

For an ideal gas, the size of a gas sample is insignificant compared to the size of the container holding the gas, and there is no interparticle interaction (no attraction nor repulsion between particles). The molar volume of the gas at a particular temperature and pressure is independent of the type of gas.

In this experiment, hydrogen gas generated by the reaction of magnesium with hydrochloric acid is collected over water.

Mg(s) + 2HCl(aq) MgCl2(aq) + H2(g)

The volume of the gas is measured and the number of moles of gas is calculated from the mass of the magnesium strip used. By dividing the volume by the number of moles we obtain the molar volume at the temperature and pressure at which the experiment is performed. In order to find the molar volume at STP, we apply the Ideal Gas Law:

P V = n R T

where P = the pressure of the gas

V = the volume of the gas

n = the number of moles of gas

R = gas constant

T = the temperature in K

Experimental Set-up

The following video provides the concept of the lab that you are going to do. Please make sure that you follow our written procedure provided to you, which is also posted below. Please take note of the following that we will NOT do which was described/demonstrated in the video:

  • try NOT to poke around with a glass stirring rod.


Dilute sodium hydroxide solution is irritating to eyes and skin. Crystal violet is a strong dye and will stain clothes and skin. Clean up all spills immediately. Wear chemical splash goggles, chemical-resistant gloves, and a chemical-resistant apron. Avoid contact of all chemicals with eyes and skin and wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Calculating Percent Water in a Hydrate

This is a video that explains the calculations.


View the procedure handout attached at the bottom of this page. Please note that we will not look at the kinetics of crystal violet, but of another, very similar molecule. The principles operate similarly to the discussion above. Apply what you learned about crystal violet to the molecules and reactions under investigation.

Access Pre-Lab Quiz here.