# What is always the same between the reactants and products in a chemical reaction?

All chemical reactions (i) conserve mass, and (ii) conserve charge. And all reaction chemistry, and stoichiometry depend on these principles. And thus if we start with $1 \cdot g$, $10 \cdot g$, $1000 \cdot g$ of reactants, we will necessarily finish with $1 \cdot g$, $10 \cdot g$, $1000 \cdot g$ of product.