an amount  Amount of substance  The amount of substance, n, of a
sample or system is a physical quantity which is proportional to
the number of elementary entities present. "Elementary entities"
may be atoms, molecules, ions, electrons, or particles, the
choice of which is dependent upon context and must be stated.
Amount of substance is sometimes referred to as chemical amount
or, incorrectly, as number of moles.  Amount of substance is a
quantity that measures the size of an ensemble of entities. It
appears in thermodynamic relations such as the ideal gas law,
and in stoichiometric relations between reacting molecules as in
the law of multiple proportions.  The SI unit for amount of
substance is the mole (symbol: mol), which is defined as the
amount of substance that has an equal number of elementary
entities as there are atoms in 12 g of carbon-12. That number is
the Avogadro constant, NA, which has a value of . The only other
unit of amount of substance in current use is the pound mole
(symbol: lb-mol.),