The molar volume of gases

A particularly useful special case: gases

For solids and liquids, the volume occupied by one mole depends greatly on the substance. But for gases, under given temperature and pressure conditions, a remarkable property emerges: one mole of any gas occupies (roughly) the same volume, regardless of its chemical nature.

The molar volume Vm

This volume occupied by one mole of gas is called the molar volume, denoted Vm, and expressed in L/mol. Under standard temperature and pressure conditions (STP: 0 °C and 1013 hPa), the standard value is:

Vm = 22,4 L/mol

The relationship to use

For a gas, the amount of substance n and the volume V it occupies (under the same conditions) are related by V = n × Vm, or equivalently n = V / Vm.

Practical example

What volume do 0.50 mol of oxygen (O₂) occupy under standard conditions?

V = n x Vm = 0,50 x 22,4 = 11,2 L

Conversely, if a flask contains 11.2 L of dihydrogen under these conditions, it contains n = 11.2 / 22.4 = 0.50 mol of gas.

Common mistake

Do not confuse the molar volume Vm (the volume of ONE mole, a characteristic of the gas under given conditions) with the volume V of any given sample, which depends on the actual quantity of gas you have. Also, be careful with units: if a volume is given in mL, convert it to L before dividing by Vm (which is in L/mol).