is: the SI unit of amount of substance, one of the seven base units. [P7.1, P7.2]
is defined: as the amount of a substance that contains the same number of elementary entities as the number of atoms in 
of the 
isotope of carbon. (Measurements show that of contain (to four significant figures) 
atoms of 
) The elementary entities may be atoms or molecules. For example, one mole of 
contains magnesium atoms and 
fluorine atoms. [P7.1, P7.2]
facilitates: the evaluation of the molar mass of a substance, the numerical value of the molar mass in grams per mole being obtained by adding together the relative atomic masses of the atoms in the molecule. The relative atomic mass of magnesium is 24.3, and of fluorine is 19.0. Thus, one mole of has a mass of 
or approximately 
[P7.1, P7.2]
See Avogadro's constant and Avogadro's number.
Copyright 1997, The Open University