James Richard Fromm
Chemical reactions which consumed and produced gases were studied carefully by many chemists at the beginning of the nineteenth century. In 1809, the French chemist Joseph Louis Gay-Lussac summarized the results of many experiments into what we now call Gay-Lussac's law of combining volumes:
When measured under the same conditions of temperature and pressure, the volumes of gases which react together are in the ratio of small whole numbers.
The measurements of volume made in 1809 were sufficiently accurate to show that the volume relationships were in fact integers. For example, one volume of hydrogen reacts with one volume of chlorine to produce one volume of hydrogen chloride; two volumes of hydrogen react with one volume of oxygen to produce two volumes of water vapor; and three volumes of hydrogen react with one volume of nitrogen to produce three volumes of ammonia.
The law of combining volumes was interpreted by the Italian chemist Amedeo Avogadro in 1811, using what was then known as the Avogadro hypothesis. We would now properly refer to it as the Avogadro law:
Equal volumes of gases under the same conditions of temperature and pressure contain equal numbers of molecules.
Avogadro's interpretation was not accepted for some forty years, during which confusion prevailed in distinguishing atoms from molecules. Elemental gases were assumed to be monatomic. We now know that most of the common gaseous elements actually exist as diatomic molecules: hydrogen, nitrogen, oxygen, fluorine, and chlorine. Avogadro's interpretation cleared up many of these discrepancies; it enabled explanation of the empirical results of Gay-Lussac in terms of simple molecular reactions:
H2(g) + Cl2(g) 2HCl(g)
2H2(g) + O2(g) 2H2O(g)
3H2(g) + N2(g) 2NH3(g)
The Avogadro law is equivalent to the statement that volume is directly proportional to number of atoms or molecules N. Since the fundamental unit of amount of substance, the mole, is equal to Avogadro's number of atoms or molecules NA, the amount of substance n, in moles, is N/NA. Avogadro's law can then be written in equation form as:
V = k"n = k"N/NA