The reliability of values of molar mass, the factor that relates measurements expressed in two SI base units (mass and amount of substance)

Progress in science is often achieved by modifications of hypotheses imposed by reliable results of measurements with small uncertainty. The application of the discipline of metrology to chemistry, based on the SI and the estimation of uncertainties, is illustrated by suggested refinements of the presentation of molar-mass values of the elements as recommended, and biennially revised, by the International Union of Pure and Applied Chemistry (IUPAC). All these values from 1969 to 1995 are tabulated here with attributed uncertainties, and the changes achieved in all these uncertainties are presented pictorially. Molar mass is the factor that converts, with its entity-specific uncertainty, the mass of that specified entity in a given sample from a value expressed as a mass quantity to one in the amount-of-substance SI base quantity. An element with two or more isotopes may exhibit differences in isotope abundances. A small range of molar-mass values may exist even in normal terrestrial materials. The IUPAC combines, with only one-digit precision, such ranges with experimental uncertainties. Nevertheless, the IUPAC values in the past have proved highly reliable for use in chemistry. For anticipated improvements in the accuracy of some chemical measurements, two-digit uncertainties, evaluated and published separately from normal terrestrial ranges of molar mass, are proposed.