Avogadro's Law states, "Equal volumes of all gases, at the same temperature and pressure, contain equal number of molecules."

Thus, according to Avogadro's law, 1 m3 of oxygen (O2) will contain the same number of molecules as 1 m3 of hydrogen (H2) when the temperature and pressure is the same. A little consideration will show, that as the molecular mass of hydrogen is 2 and that of oxygen is 16, therefore a molecule of oxygen has a mass which is 32 / 2 = 16 times the mass of hydrogen molecules. Moreover, as 1 m3 of these two gases contain the same number of molecules, and a molecule of oxygen has a mass 16 times than that of hydrogen molecule, therefore it is evident that density (or specific mass) of oxygen is 16 times the density of hydrogen. Hence, the Avogadro's law indicates that the density (or specific mass) of any two gases is directly proportional to their molecular masses, if the gases are at the same temperature and pressure.

Note: The molecular mass expressed in g (i.e. 1 g- mole) of all gases, at normal temperature and pressure (briefly written as N.T.P.), occupies a volume of 22.4 litres.