Level: introductory
References: Jean S. Stas, "Researches on the Mutual Relations of Atomic Weights," Bulletin de l'Académie Royale de Belgique [2] 10, 208-336 (1860) as translated and excerpted in Alembic Club Reprint #20, Prout's Hypothesis (pp. 208-213, 336 included)
Jean Charles de Marignac, "Researches on the Mutual Relations of Atomic Weights by J. S. Stas," Bulletin de l'Acad. Royale de Belgique, [2] 10, No. 8. [Reprinted and translated in its entirety in Alembic Club Reprint #20, Prout's Hypothesis from Bibliothèque Universelle (Archives) 9 (1860), 97-107.]
Stas did the experiments and published the results; presumably data may be found in his original paper. The excerpt of Stas's paper in the Alembic Club Reprint and on the internet, however, omit the data and include only conclusions and interpretation. Marignac's paper, which is a commentary on Stas's, includes a summary of the data. That summary appears in the Alembic Club and internet excerpts, and that is where I drew the data for these exercises.
Notes: Jean Servais Stas (1813-1891) was a careful chemical analyst, best known for the careful determination of atomic weights. The exercises here reproduce the calculations Stas would have done, or their equivalents, to extract atomic weights from his analytical data. Most of the following terminological and historical fine points are not appropriate for an introductory course, although the basic stoichiometric manipulations involved in the exercise are.
Exercise 2 reflects the ambiguity that beset atomic weight determinations even as late as 1860: formulas were needed to determine atomic weights, but atomic weights were needed to determine formulas. In most of the calculations involving formulas, composition, and stoichiometry in Classic Calculations, I have chosen to avoid the confusion of this ambiguity by directing students to a modern source of atomic weights, but this exercise highlights the ambiguity.
Atomic weight determinations of this time and earlier depended on sometimes uncertain formulas and also an arbitrary basis value for the atomic weight of one element. Common bases were H = 1, O = 16, C = 12, or (the one used here), Cl = 35.5. All of these bases give results quite close to the atomic weights that appear in modern sources (which are, in fact, based on 12C = 12). Other early 19th-century compilations used O = 10 or O = 100.
Another ambiguity I have treated here differently than in most other Classic Calculations has to do with units and terminology. Elsewhere, I usually use the term molar mass and pose problems in such a way that they are most straightforwardly solved using the mole concept--even though that concept was not yet invented or well defined at the time the data were collected. Also, I usually treat mass ratios as if they were masses in particular units, usually grams. Molar mass is, of course, the mass of one mole, a macroscopic quantity; it is typically expressed in units of g/mol. Atomic weight or molecular weight (or sometimes atomic or molecular mass) is now defined to apply to atoms or molecules (i.e. to microscopic quantities), but historically the terms were also used to refer to the relative weight of macroscopic quantities. Now atomic weights are given in atomic mass units (amu) defined as 1/12 the mass of a 12C atom; historically they were often expressed without units, or with a unit implied. Modern definitions of the mole and atomic weights are such that a molar mass expressed in g/mol is numerically equivalent to an atomic or molecular weight expressed in amu. This fact is frequently not realized by students when they have occasion to put the actual mass or an atom or molecule in MKS units into a mathematical relationship in, say, quantum mechanics. At any rate, in these exercises, I refer to atomic weights rather than molar masses, and I present Stas's data as mass ratios rather than absolute masses.
Historical note: The motivation for Stas's researches was Prout's hypothesis (1815) that atomic weights were integer multiples of that of hydrogen. If so, hydrogen might somehow be a building block of the other elements. As atomic weight determinations grew more precise, Prout's original hypothesis became untenable. For example, the atomic weight of chlorine was pretty close to 35.5 and certainly not 35 or 36. Still, a modified version of the hypothesis persisted: maybe hydrogen was not the unit but maybe 1/2 or 1/4 of hydrogen was the unit. Although this also was not correct (in fact, it was further from the truth than Prout's original hypothesis) it inspired accurate determinations such as Stas' and, 25-30 years later, the investigations of Lord Rayleigh that eventually led to the discovery of argon.
Jean Charles de Marignac (1817-1894) was also an analytical chemist and a contemporary of Stas. Marignac discovered ytterbium and gadolinium.
Solutions: To download solutions, go to:
http://web.lemoyne.edu/giunta/classicalcs/stas.doc
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