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Avogadro's law states:

• equal volumes of different gases contain an equal number of particles under the same conditions of temperature and pressure

This means that equal numbers of moles of gases occupy the same volume under the same conditions of temperature and pressure. For example:

• 1 mol of hydrogen gas occupies the same volume as 1 mol of methane gas
• 10 mol of nitrogen monoxide gas occupy the same volume as 10 mol of argon gas
• the volume of 1 mol of any gas under these conditions is 24 dm³

Avogadro's law can be used to calculate the volumes of gases involved in reactions.

Using Avogadro's law

Hydrogen reacts with chlorine to form hydrogen chloride:

H₂(g) + Cl₂(g) → 2HCl(g)

The mole ratio of hydrogen to chlorine is 1:1. This means, for example, that in terms of volume:

• 1 cm³ of hydrogen reacts with excess chlorine to produce 2 cm ³ of hydrogen chloride
• 250 cm³ of hydrogen reacts with excess chlorine to produce 250 cm³ of hydrogen chloride

The mole ratio of hydrogen to hydrogen chloride is 1:2. This means, for example:

• 1 cm³ of hydrogen produces 2 cm³ of hydrogen chloride
• 250 cm³ of hydrogen produces 500 cm³ of hydrogen chloride