Mole concept and molar mass

Because even a very small, weighable amount of any substance contains an enormous number of atoms or molecules, chemists borrow the same idea behind everyday counting words like 'dozen' (12 items) or 'gross' (144 items) and use the word 'mole' as a quantitative adjective for these very large numbers of submicroscopic particles — atoms, ions, electrons, or molecules — present in a sample. Formally, one mole is defined as the amount of a substance that contains as many elementary entities (particles) as there are atoms in exactly 12 g (0.012 kg) of the carbon-12 isotope. Since the mass of a single carbon-12 atom (found using a mass spectrometer) is 1.992648 x 10^-23 g, and one mole of carbon atoms has a mass of 12 g, the number of atoms present in 12 g of carbon-12 works out to 12 g/mol divided by 1.992648 x 10^-23 g/atom, which equals 6.0221367 x 10^23 atoms per mole — a number known as Avogadro's constant, symbol NA, named in honour of Amedeo Avogadro, and the same number of particles present in ONE mole of any substance, whether that substance's particles are atoms, molecules, or formula units (for example, one mole of oxygen atoms, one mole of water molecules, and one mole of sodium chloride formula units, all contain 6.0221367 x 10^23 of their respective particles). The mole (symbol 'mol') was itself formally introduced into the SI system as its seventh base quantity, used to express the amount of a substance. Building on this, the molar mass of a substance (element or compound) is defined as the mass, in grams, of exactly one mole of that substance. For a single element, this molar mass in g/mol is numerically identical to that element's atomic mass expressed in u (see the accompanying comparison t …