Impact of the molar mass constant: the molar mass and its associated uncertainty

Abstract After the revision of the International System of Units—the SI, the determination of the molar mass of an element or a compound is affected by the molar mass constant M u and its associated uncertainty u ( M u ). The variations and changes of the molar masses of silicon crystals highly enriched in 28 Si used in the x-ray crystal density method for the realization and dissemination of the mole and the kilogram were investigated and a respective overview is given. All molar mass values M of a certain crystal agree within their uncertainties over the last years after the SI revision. The relative changes of M compared to Committee on Data for Science and Technology M (CODATA 2014) with u ( M u ) = 0 g mol −1 are −3.5 × 10 −10 (CODATA 2018) and +1.05 × 10 −9 (CODATA 2022). The experimentally determined M u is mainly dependent on the fine structure constant α . A theoretical impact of M u and u ( M u ) is simulated for a highly enriched silicon crystal according to the ‘Guide to the Expression of Uncertainty in Measurement’. The simulation shows no significant change with the precondition of a similar development of α in the next years.