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In contrast with some other factors that relate Système International (SI) quantities, those involving amount of substance vary by entity in their magnitude and their uncertainty. The mole-to-gram factor, called molar mass, like the mole itself, has to be defined by chemical formula, at times also by isotropic composition of constituent elements of a compound in a sample. Chemistry has long depended greatly on the molar-mass values of the chemical elements in their natural terrestrial occurrence. The determination of these data has called for great skill in chemical analytical methods until isotope-ratio mass spectrometry provided values with smaller uncertainties. The best current knowledge of the set of molar-mass values is published biennially by the International Union of Pure and Applied Chemistry (IUPAC). Since 1969, from which time these uncertainties were systematically estimated, this data set has become highly reliable within the uncertainties given. It must be understood, however, that the IUPAC uncertainties include estimated variability of molar-mass values of the elements in their normal occurrences.

Additional information

Authors: DE BIÈVRE P, JRC Geel (BE);PEISER H S, NIST, Gaisthersburg (US)
Bibliographic Reference: Article : Metrologia (1996)
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