## Gay-Lussac multiple proportions

Content: composition, multiple proportions, stoichiometry

Level: introductory

Reference: Joseph Louis Gay-Lussac, "Memoir on the Combination of Gaseous Substances with Each Other," Mémoires de la Société d'Arcueil 2, 207ff (1809)

Notes: Joseph-Louis Gay-Lussac (1778-1850) carried out important research on the physical and chemical properties of gases. He helped establish that the volume of a gas at constant pressure varies simply with its temperature (Gay-Lussac 1802), a result known today as Charles' law. His study of gases was not confined to the laboratory: he ascended several thousand meters in a baloon to make meteorological and magnetic measurements, and to collect samples of gas for chemical analysis. His work on the acids hydrogen fluoride, hydrogen chloride, hydrogen iodide, and hydrogen cyanide led to the realization that acids need not contain oxygen. He was also first to isolate the element boron. This exercise deals with the combining volumes of gases, that is to the ratios by volume in which gases react.

Gay-Lussac's main conclusion in this paper is that gases combine in simple ratios by volume. Some of the data presented in it also support the law of multiple proportions, which had just recently come to light. The law of multiple proportions states that when two elements combine to form more than one distinct compound, the combining ratios of the elements in the various compounds are simple whole-number multiples of each other. This law is obvious if molecules are composed of small whole numbers of atoms, as we know them to be. Amedeo Avogadro built upon Gay-Lussac's paper on combining volumes and John Dalton's atomic notions to give the molecular picture of simple gas-phase reactions still employed today. Since chemical reactions take place among small whole numbers of molecules, a version of the law of multiple proportions also applies to reactions that take place in more than one stoichiometric ratio between the same reactants (multiple proportions in stoichiometry as opposed to in composition). The third exercise in this set illustrates this corollary.

Further information: A detailed summary of key primary literature on multiple proportions, the atomic hypothesis, and atomic weights, including some quantitative treatment of data may be found in Leonard Nash, "The Atomic-Molecular Theory," in James Bryant Conant, ed., Harvard Case Histories in Experimental Science, vol. 1 (Cambridge, MA: Harvard, 1957), pp. 215-321.