Chemical Education Today
Letters Correctly Expressing Atomic Weights
The authors reply:
In an article having the above title (J. Chem. Educ. 2000, 77, 1438) it is proposed that the traditional (nondimensional) definition of [atomic and] molecular weights be altered, so that they would be expressed as dimensional quantities. I disfavor this proposal, and want to point out that there is an alternative to it, which is very simple and completely straightforward. Anyone who does not wish to deal with unitless atomic and molecular weights has the option of multiplying the numbers in question by the SI-derived unit (1 g/mol). The result of this operation is conventionally called “molar mass”, M; it is defined by the equation, M = m/n, in which m represents mass, and n the so-called “amount of substance”, or “chemical amount” (1). This relationship is general, inasmuch as the definition of the SI mole stipulates that the molar mass of carbon-12 equals exactly 12 g/mol. So, for example, if the molecular weight of a given protein is 50,000, its molar mass equals 50,000 g/mol, and it must consist of molecules, each of which has a mass of 50,000 daltons. By this approach, one can choose whichever description of the system is most convenient and appropriate to the context.
While agreeing with the reasoning expressed in the letter by George Gorin, we would like to underline the following points. His starting point is that “the numbers in question” are to be multiplied by 1 g/mol in order to obtain the molar mass or mass of a mole, and by 1 dalton/molecule to obtain the mass (in daltons) of a molecule: it is clear that the “numbers in question” are the molecular weights expressed as pure numbers. By contrast, we discussed the possibility of expressing molecular weights either by pure numbers, or by the dalton unit (according to a more general way of expressing quantities) (1). Adopting the second point of view, the molar mass M is given by
Literature Cited 1. International Union of Pure and Applied Chemistry. Quantities, Units and Symbols in Physical Chemistry, 2nd ed.; Mills, I.; Kallay, N.; Kuchitsu, K.; Cvitas, T.; Homann, K., Compilers; Blackwell: Oxford, 1993; p 41. George Gorin Chemistry Department Oklahoma State University Stillwater, OK 74078
[email protected] M = mass of a molecule daltons × molecule grams grams Avogadro no. molecules × = mole dalton mole The two points of view are equally tenable because they stress different requirements: (i) the approach of the SI system whereby a single unit of measurement is recommended (e.g. the gram rather than the dalton for mass); (ii) the principle of giving to each quantity a dimensional unit of measure (unless the quantity is essentially relative, as for example in the case of absorbance). Literature Cited 1. Paolini, M.; Cercignani, G.; Bauer, C. J. Chem. Educ. 2000, 77, 1438–1439. Moreno Paolini Department of Pharmacology, Biochemical Toxicology Unit University of Bologna, I-40126 Bologna, Italy
[email protected] Giovanni Cercignani and Carlo Bauer Department of Physiology and Biochemistry Biochemistry Unit University of Pisa, I-56100 Pisa, Italy
JChemEd.chem.wisc.edu • Vol. 79 No. 2 February 2002 • Journal of Chemical Education
163
