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1 mol of any gas at 20°C and 1 atm pressure occupies a volume of 24 dm³ (which is the same as 24 000 cm³). This volume is known as the molar gas volume (V_m).

The volume of a gas may be calculated from its number of moles using:

volume of gas = moles × V_m

rearranging this:

The volume of gas and the V_m are always in the same units. If you are using a gas volume in cm³, then V_m = 24 000 cm³. Similarly if the V_m you use is 24 dm³ then the gas volume calculated will be in dm³.

Example: Calculate the volume of hydrogen gas produced in cm³ when 0.3 g of magnesium ribbon react with excess hydrochloric acid.

(A_r of Mg = 24)

Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

moles of H₂ = 0.0125 mol (as 1:1 ratio of Mg:H₂)

volume of H₂ = moles × V_m = 0.0125 × 24000 = 300 cm³