The mole is the
The mole is widely used in chemistry as a convenient way to express amounts of reactants and products of chemical reactions. For example, the chemical equation 2 H2 + O2 → 2 H2O implies that 2 mol
The term gram-molecule was formerly used for essentially the same concept.  The term gram-atom has been used for a related but distinct concept, namely a quantity of a substance that contains Avogadro's number of atoms, whether isolated or combined in molecules. Thus, for example, 1 mole of MgBr2 is 1 gram-molecule of MgBr2 but 3 gram-atoms of MgBr2.  
As of 2011, the mole is defined by International Bureau of Weights and Measures to be the amount of substance of a system which contains the same number of constitutive entities (e.g.
The number of constitutive entities in a sample of a substance is technically called its
The mass of one mole of a substance depends not only on its
Since the definition of the gram is not (as of 2011) mathematically tied to that of the atomic mass unit, the number of molecules per mole NA (the Avogadro constant) must be determined experimentally. The value adopted by
The number of moles of a sample is the sample mass divided by the molar mass of the material.