One of the more common mistakes a student makes in stoichiometric calculations is interpreting the chemical reaction as a mass rather than mole relation. For example, consider the oxidation of iron metal with molecular oxygen:
A beginning student might incorrectly interpret this equation from the perspective of a mass relation: 2 grams of iron reacts with 3 grams of oxygen to produce 1 gram of iron(III) oxide. Clearly this interpretation is wrong as it would result in a violation of the principle of conservation of mass. The correct interpretation if this equation is the mole-mole relation:
This mole-mole interpretation is the proper interpretation for all balanced chemical equations.