The Definition and Symbols for the QuantityCalled "Molarity" or "Concentration" and for the SI Units of this Quantity In most American textbooks, the name "molarity" 1s applied to a measure of the composition of solutions, which is described with reference to a particular unit of measurement, the mole per liter, (mol/L).' A definition of this type poses some difficulty, because it is inconsistent with the method that the books recommend for calculating other quantities, which places no restrictions on what units may be used. The purpose of this note is to call attention to an alternative formulation for the quantity in question and to discuss related points of nomenclature and symbolism. There are more than a dozen different ways of describing composition of solution^,^ but many of them are seldom used in practice; attention should therefore be focused on those that are. The quantity most useful to, and most frequently used by, chemists may be defined hy eqn. (I),
in which Vrepresents the volume of the solution, and ns is the so-called "amount of substance," or "chemical amount," of a soluteB. If V is measured in liters and ne in moles, it follows this general equation that the units of quantity [B] will he (mol/L); this is the unit most often encountered in ~ractice.However. other units are not orecluded.~,for easmole those mentioned in the following equalities: ~
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Equation (1) is given in the "Manual of Symhols . .,"recommended by the competent committee of IUPAC.3 This manual also recommends that the quantity [Bl he called "amount of suhstance concentration," with the proviso that "concentration" alone may he used "when there is no risk of ambiguity." These recommendations pose a dilemma because there is, almost always, a risk of confusion with the quantity "mass concentration (of B)" = p s (mass of solute B)/(volume of solution). In these circumstances, what are teachers to do? They should stress the point that the definition expressed by eqn. (1) is unambiguous, regardless of what name is applied to quantity [B]; then they should use whichever designation they like best. In descrihing specific values of the quantity [B], English-speaking chemists frequently employ the adjective "molar" and the symbol M i n contexts such as the following: A solution in which, say, [B] = 1.023 m o l L may he described by the symbol 1.023 Mand, in speech, a8"onepoint zero two threemolar." This usage of molar and Mis not recommended by IUPAC;' but it seems unlikely that chemists will soon he persuaded to abandon this convenient notation. Again, what are teachers to do? Clearly they should teach students properly to interpret what this notation means, and perhaps they should add a warnina that the IUPAC recommendations are different.5 In prinrible, nearly ewryone agrees aith the proposition that we should haw, in arienrr, a precise end unambiguous nomenclaturr. Cnfortunstels, that ideal in not ensily attained. The present case is an example.
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A representativegroupof 10 recent, widely used texmwks have been examined anda paper on Vntresulta has been presented(Gorin, G.. a d Freeman, R. D.. "Abstracts. 187lh National Meeting." CHEDlO8, St. Louis, MO. April 1984): copiesafthe paper may be obtainedon re. quest. The textbwks examined were: Allen. Keefer (1982) ISBN &06020209-1: Bailar et ai. (1984) 0-12-072855-9: Brady. Humistan (1982) 0-471-07808-9: Brown. LeMay (1981) &13-1285041: Chang (1984) 0-39432983-X: Hoitzclaw et ai. (1984) 0-669-06333-9: Mastertonet al. (19831 0-03-0626463: Martimer (1983) 0-53401184-5: Petrucci (1982) 0-02-395010-2: Whitten. Gailey (1984) 0-03-060458-3. The relationship between mlarity and (moi/L) is stated explicitly in most cases. but not in all. in nine of the books, moiarity is the only or the prafened name given for the quantity under discussion; in one case. "molar cancenhatian" is preferredand maiarity given as a synonym. Twelve such quantities have been described and classified by McCarthy, P.. J. CHEM.EDUC..60, 187 (1983). When a q u a n t i is defined so that only one unitwill be used to describe it, the distinctionbetweenthese two concepts becomes blurred, and in that article the quantities malarity, mass concentration. etc. are called "concentration units." Commission on Symbals. Terminology and Units, International Union of Pure and Applied Chemistry, Pure App. Chem.. 51, 1;#2.3.11 and footnote 4. #2.3.12. (1979). 'Footnote (3). pp. 6. 19. it must be admittedMat mere is no great advantageto using Minsteadof (mollL) and that the ianer makes calculations clearer. For exampie, the value of % in 10.11 mLof a 1.023 Msoiution is given by: rg = 10.11 mL X 1.023 mmoi/mL = 10.34 mmal.
George Gorin Oklahoma State University Stillwater. OK 74078
Volume 62
Number 9
September 1985
741
