Amount of Substance
mole, mol: The mole is the amount of substance in a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Quantitative studies of chemical analyses and reactions are most conveniently understood in terms of the ratios of the number of atoms or molecules involved. Unfortunately the number of molecules in any practical sample of a substance is huge, of the order of 6 x 1023. The mole provides an alternative measure of the amount of substance that is much less unwieldy. The practical realisation of the mole benefits greatly from our ability to measure the ratio of atomic masses to very high accuracy. Measurements of the mass of a sample then allow us to infer the number of moles in a pure sample simply.
Whereas determining the number of moles in a pure sample is easy, determining the amount of substance in impure or dilute samples is extraordinarily difficult. Problems associated with ongoing chemical reactions, imperfectly mixed samples, degradation of the sample, and the influence of other, perhaps similar, chemical species are influence variables unlike those found in physics. Also the huge range of chemical species make chemical metrology the broadest of the metrological fields.