For the reaction, #A+BrarrC#, clearly there must be an equal number of atoms (or molecules) of #A# and #B#; if these undergo complete reaction, an equal number of #C# particles result.
If there are not equal numbers of #A# and #B#, then ONE of these reagents will be in excess, and ONE will be in deficiency. Since the stoichiometry of the reaction demands 1:1 equivalence, the reagent in excess is along for the ride and will not undergo reaction.
This leads to the calculation of the number of moles of #A# and #B#. The mass of an atom or molecule corresponds to the NUMBER of those molecules. That is why chemists speak of equivalent masses. The masses ARE different, but these different masses will contains the SAME number of particles. (SAYS he for some reason capitalizing the words HE thinks ARE important!).
