Suppose a chemist wishes to carry out the chemical reaction of adding bromine to hexene to give dibromohexane:
C₆H₁₂ + Br₂ C₆H₁₂Br₂
starting with a known amount of hexene.

How does the chemist know how much bromine is needed?

The chemical equation tells you that one molecule of dibromine is required for each molecule of hexene. But the very large number of molecules required for reactions on a practicable scale cannot be counted. If the mass of hexene is known how can the mass of bromine required be calculated?

This section tells how this type of question is answered. It requires the introduction of a new quantity specific to chemistry - amount of substance and its unit the mole.