Level: introductory
Reference: quoted in Ida Freund, The Study of Chemical Composition (New York: Dover, 1968) [first published in 1904], p. 166.
Notes: Jöns Jakob Berzelius (1779-1848) was a meticulous analytical chemist, and two of his analyses are the subject of this set of exercises. Berzelius was in many ways the most influential chemist of the first half of the 19th century. He developed a dualistic theory of chemical bonding, based on the electrical polarities of elements. He introduced the concepts of allotropy, catalysis, and isomerism. He discovered or isolated three elements, selenium, silicon, and thorium. He corresponded with chemists throughout Europe and produced an "annual reviews" volume for many years. And he proposed the system of elemental symbols still in use today.
Berzelius compiled quite an accurate table of atomic weights. Rather, his atomic weights were based on meticulous analyses, but not always on correct valences. (Indeed, the concept of valence was not yet around. However, it is easy to see that if Berzelius believed that the mass in a chemical analysis was due to two atoms in a compound rather than one, the atomic weight that would result would be half of the accurate value.) Exercises 1 and 2, then, avoid some of the difficulties of interpretation that Berzelius found in chemical analyses, and gives students the benefit of the later developments of the mole and atomic weights based on correct valences.
Exercise 3, though, is one that Berzelius could have done. What his wet chemical analyses produced were the masses and mass ratios of elements in compounds. It provides an example of the law of multiple proportions, which had been proposed in the earliest years of the 19th century. Indeed, chemical historian Ida Freund used the analyses discussed here as one example of the growing body of evidence obtained in the 19th century for that law. (Freund's book, originally published in 1904 and reprinted in 1968, is a vast compilation of data; however, references to the original sources of those data are spotty at best.)
Instructors may wish to reinforce the law of multiple proportions through the combining masses (as in exercise 3) or the empirical formulas (obtained in exercises 1 and 2). Of course the molecular interpretation of formulas reflects compositions in terms of atoms, and it is the notion of elements combined atom to atom that underlies the law of multiple proportions.
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://web.lemoyne.edu/giunta/classicalcs/berzelius.doc
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