Avogadro Law

Avogadro's law, proposed by Amedeo Avogadro in 1811, states that equal volumes of any gas, measured at the same temperature and pressure, contain an equal number of molecules — regardless of what the gas actually is. This single idea neatly explains why Gay-Lussac's gas-volume ratios always came out as simple whole numbers: if one 'unit volume' of any gas always corresponds to the same fixed number of molecules — call it n — then in the hydrogen-oxygen-water reaction, 2 volumes of hydrogen (i.e. 2n molecules) combine with 1 volume of oxygen (i.e. n molecules) to give 2 volumes of water vapour (i.e. 2n molecules), with no oxygen left over unreacted — matching the observed 2:1:2 volume ratio exactly. Making this reasoning work correctly required Avogadro to recognise something genuinely new for his time: that gas molecules of elements like hydrogen and oxygen are themselves made of more than one atom bonded together (i.e. are diatomic, …