Amount of substance
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The amount of substance of an object is a physical quantity that measures the number of elementary entities in the object. "Elementary entities" may be atoms, molecules, ions, electrons, or particles, the choice of which is dependent upon context.
The SI unit for amount of particulate substance is the mole (mol) which is defined to be the amount of substance that has an equal number of elementary entities as there are atoms in 0.012 kilogram (or 12 grams) of pure carbon-12. That number is the Avogadro constant which has a current 2002 CODATA measured value of 6.0221415×1023 ± 0.0000010×1023 mol-1.
The amount of substance is not the same as the amount of matter (mass) but, for a particular substance, is related to mass by a factor known as the molar mass.
