The Unit Gram/Mole and Its Use in the Description of Molar Mass "Molar mass" has been formally defined as the quotient of the quantities mass divided by "amount of substance."' The latter designation is not widely usedZ and is liable t o misinterpretation; for these reasons, the writer has suggested elsewhere that this quantity be called "chemical a m ~ u n t . "There ~ is, in any case, no ambiguity concerning the role of the quantity in question in the SI: i t is measured by reference t o the base unit "mole," which is the magnitude of "amount of substance," or "chemical amount," present in exactly 12 g of carhon-12.'It follows that to describe molar mass in SI units one must use the kilogram per mole, kglmol, or its multiples and submultiples. By the above definition, the molar mass of carbon-12 is 0.012 kglmol = 12 glmol, and the molar masses of other substances are in proportion. How is molar mass related to the quantity called "molecular (or atomic) weight"? Traditionally, this so-called "weight" has been described by a number, without reference ta any measurement unit.5 If the molecular (or atamic) weight of a chemical substance is described bysome number z,its molar mass isx glmol. Inmany textbooks2 anefinds, instead, reference to the "unified atomic mass unit," symbolized by u or amu, which has been defined as equal t o (1112) of the mass of the nuclide W.' It is evident that this nan-SI unit may be regarded as equivalent to the (glmol), i.e., if the molar mass of a substance is x glmol, the (average) mass of its constituent molecules or atoms is x u. These relatmnships are general and exact, because they follow from the definitions of the respective quantities and units.
' Wuantities and Units of Physical Chemistry and Molecular F%ysics." (International Standard IS0 31-Vlll-1973(E)), nos. 8 3 . 1 and &
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S . 1
*&in, G.. and Freeman. R. D., Abstracts. 187th ACS NationaiMeeting, CHED-106. St. Louis. MO. April 1984. Gorin. G.. J. CHEM. E m . . 59. 508 (1982); 60. 782 (1983). "'The International System of Units." (1981 ed.), Nat. Bur. Stand (U.S.). Spec. Pub!., 330, pp. 4, 13. sCDmmi~~ion an Atomic Weights and Isotopic Abundances, InorganIcChemistryDivision, internationalUnion of Pure and Applied Chemistry, Pure Appi. Chem.. 52,2351 (1980). 'The equivalence between (glmol)and (u) has been pointed out previously by Nelson. R. A,. J. CHEM. E m . . 56,662 (1979).
G. Gorln Oklahoma State University Stillwater. OK 74078
Volume 61
Number 12
December 1984
1045
