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Avogadro's law states that equal volumes of different gases contain an equal number of molecules.

This applies when the temperature and pressure stay the same.

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:

• 1 cm³ of hydrogen reacts exactly with 1 cm³ of chlorine
• 250 cm³ of hydrogen reacts exactly with 250 cm³ of chlorine

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