# Why is a balanced chemical equation needed for stoichiometric calculations?

Because by definition stoichiometry means $\text{in the right measure}$. If you don't have a balanced chemical equation your proportions will be out!
Mass is conserved for every chemical process and equation, whereas it may not be conserved for every nuclear reaction. Every reactant particle must thus correspond to a product particle; and if you start with $10 \cdot g$ with respect to reactants, there must be $10 \cdot g$ with respect to products. If not, someone (you) has made a mistake.