Stoichiometry

A balanced chemical equation is a recipe written in particles. In 2H₂ + O₂ → 2H₂O the numbers say two hydrogen molecules react with one oxygen molecule; scaled up, two moles of H₂ react with one mole of O₂. They do not say two grams with one gram, and reading coefficients as masses is the error that derails most first attempts at a calculation.

That is why every stoichiometry problem has the same three-step shape. Convert what you are given from grams to moles using molar mass; apply the mole ratio from the balanced equation; convert the answer back from moles to grams. The mole is the only currency the equation accepts, so it has to be entered and left at each end.

The other half of the subject is what happens when the amounts supplied do not match the recipe. Reactions stop when any one ingredient runs out, and that ingredient — the limiting reagent — alone determines how much product forms. Everything else is left over in excess. Adding more of a reactant that is already in excess changes nothing at all, which is why "double the input, double the output" fails as often as it works. Identifying the limiting reagent means comparing moles available against moles required, never masses, and it is the step that separates a correct answer from a plausible one.