Answer
Not all mass of the reactants may be consumed such as by a limiting reactant.
Work Step by Step
If we do not have stoichiometric ratios of reactants to completely consume all reactants, not all reactants will be completely consumed. The limiting reactant will be completely consumed and form products. However, other excess reactants will not be completely consumed and not completely form products. Therefore, there may be reactants left over along with the products.
