Dalton multiple proportions

Content: gas laws, multiple proportions, stoichiometry

Level: introductory

Reference: John Dalton, "Experimental Enquiry into the Proportion of the Several Gases or Elastic Fluids, Constituting the Atmosphere," Memoirs of the Literary and Philosophical Society of Manchester 1, 244-58 (1805)

Notes: John Dalton (1766-1844) is best known for formulating an atomic theory that proved to be enormously fruitful, even though it turned out to be incorrect in several important details. His theory pictured chemical compounds much as we do today, as atoms of different elements bound together. The laws of definite proportions and multiple proportions follow naturally from such an atomistic view of chemical combination. Dalton focused on properties of atoms that were amenable to empirical investigation, namely their relative masses. The fruitfulness of Dalton's theory can be seen in the research on combining masses and relative masses of the elements and their compounds that followed it. As valuable and fruitful as Dalton's work certainly was, it was mistaken in several details. Dalton believed that all atoms of the same element were identical. He also seems to have believed that they were indestructible.

Dalton is also credited with providing the earliest example of the law of multiple proportions, which is the basis of the current exercise. The observation by Dalton quoted in this exercise was of multiple proportions of combination; that is, it was an observation concerning reaction stoichiometry. As such, it is not identical to the law of multiple proportions as usually defined and taught in introductory chemistry, which is a law concerning the elemental composition of compounds. Of course multiple proportions of elemental composition and multiple proportions of reaction stoichiometry are intimately related, and both are easily explained by the discrete nature of atoms and molecules. Still, it is worth noting that Dalton observed that the proportion of nitrous gas that reacts with oxygen in one reaction is twice as great as in another, not that the ratio of oxygen bound to nitrogen is 3/2 as great in one product as in the other.

The exercise does not reproduce Dalton's computations. Rather it takes some observations that were noteworthy because they illustrate multiple proportions in reaction stoichiometry, and asks the students to express the chemical quantities and the reactions in the modern chemical vocabulary of moles, formulas, and chemical equations.

One final gloss on Dalton's observations. The reactions were carried out in a vessel inverted over water, so they depended on the fact that the reactants are essentially insoluble in water but the products soluble. So Dalton could see that when 36 units of (insoluble) nitrous gas were added to 100 units of (insoluble air), the result was 79-80 units of nitrogen (originally present in the air, unreactive, and insoluble). He did not characterize the products of the reaction, but he understood that the water must have absorbed (dissolved) them.

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.

Solutions: To download solutions, go to:
http://webserver.lemoyne.edu/faculty/giunta/classicalcs/daltonmult.doc


Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.

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