A balanced chemical equation is the foundation of stoichiometry: because atoms are neither created nor destroyed (the law of conservation of mass), the coefficients give the exact mole ratio in which reactants combine and products form. For the combustion of methane, , the coefficients say that exactly 1 mole of reacts with 2 moles of — a ratio fixed in moles, never in mass. Stoichiometry is the set of calculations that uses this mole ratio to connect the amount of any one substance in the equation to the amount of any other, by converting given masses to moles, applying the ratio, and converting back to whatever unit is asked for.
Real reaction mixtures are rarely combined in the exact ratio a balanced equation demands. When reactants are combined in any other ratio, one of them is used up first, stopping the reaction and leaving some of the other reactant unconsumed — the one consumed first is the limiting reagent, and it alone fixes how much product can form, however much of the other, excess reagent remains. To find it, the amount of each reactant supplied is converted to moles and divided by its own coefficient in the balanced equation; whichever reactant gives the smallest such value is the one that runs out first.
For , supplying nitrogen and hydrogen in any mole ratio other than exactly 1 : 3 guarantees that one of them is limiting. Every downstream calculation — moles of formed, and mass of the excess reactant left unreacted — must then be built from the limiting reagent alone, never from whichever reactant's amount was given first or looks larger.
Because real reactions rarely proceed with perfect efficiency, the mass of product actually recovered (the actual yield) is usually less than the mass calculated from the limiting reagent (the theoretical yield). Their ratio, expressed as a percentage, is the percentage yield: (actual yield / theoretical yield) × 100.
The classic NEET trap is identifying the limiting reagent by comparing the raw moles (or raw masses) of the reactants directly, without dividing by their coefficients first. A reactant present in fewer moles is not automatically limiting if the equation also requires less of it — in , having more moles of than can still leave hydrogen as the limiting reagent, because each mole of demands three moles of . Only after scaling each reactant's moles by its own coefficient can the two be compared on common ground.