on the left represents the reaction between hydrogen and chlorine
gases to form the strong acid, hydrochloric acid. All the reactants
are in the right proportion, they will all be used up.
on the right represents a more complex chemical reaction. Two nuts,
two washers and two bolts form two units of product.
at the reactants on the left it is obvious that some reactants are
in excess. The reactant that will be used up is the bolt. Both bolts
will be used up and therefore the bolts are known as the limiting
of product (bolts+nuts+washers) are formed. The excess reactants (4
nuts and 2 washers) remain unreacted.