Definition of Avogadro's hypothesis

Avogadro's law states that 'under similar conditions of temperature and pressure, equal volumes of all gases contain equal number of molecules'.

For example, if equal volumes of different gases like hydrogen, oxygen and chlorine are held in separate containers under similar conditions of temperature and pressure, then each container will contain the same number of molecules. The total mass and size of each gas in the container may differ from each other.

Fig:1.5 - Different gases under same conditions of pressure and temperature