The chemical reaction equation indicates directly the number of atoms or molecules taking
part in the reaction. Avogadro constant, NA, when expressed in the unit mol-1 and is called the
Avogadro number. The amount of substance, symbol n, of a system is a measure of the
number of specified elementary entities. An elementary entity may be an atom, a molecule, an
ion, an electron, any other particle or a specified group of particles. The mass of 1 mole of a
substance is equal to its relative atomic or molecular mass in grams. The atomic mass of an
element gives us the mass of one atom of that element in atomic mass units (u).

1 g hydrogen has​