Reactions run until one reactant is fully consumed, and after that point no more product can form no matter how much of the other reactant remains. The reactant that runs out first is the limiting reagent, and it alone determines how much product the reaction can theoretically produce.
Finding the limiting reagent
Divide the moles available of each reactant by its coefficient in the balanced equation. The smaller result identifies the limiting reagent — it is the one closer, proportionally, to running out.
For example, in 2H₂ + O₂ → 2H₂O with 4 mol H₂ and 1 mol O₂ available: H₂’s ratio is 4/2 = 2, O₂’s ratio is 1/1 = 1. Oxygen’s ratio is smaller, so oxygen is limiting — despite there being far fewer moles of it in absolute terms. Comparing raw mole counts without dividing by the coefficients is the single most common mistake in this topic.
Theoretical yield
Once the limiting reagent is known, its available moles (divided by its own coefficient) set the “extent of reaction” — how far the reaction can proceed. Multiplying that extent by the product’s coefficient gives moles of product, and multiplying by the product’s molar mass converts that to a mass. This is the absolute maximum the reaction could produce under ideal conditions.
Why actual yield is always lower
Real reactions rarely hit their theoretical yield. Side reactions consume some reactant into unwanted products. Reactions may not go to completion, especially reversible ones that reach equilibrium before all the limiting reagent is used. Purification steps — filtering, recrystallising, distilling — lose some product at every stage. And simple mechanical loss, product left behind on glassware or lost during transfer, chips away at the total.
A percent yield above 100% is a red flag, not a triumph — it almost always means the product still contains impurities such as residual solvent or an unreacted starting material, adding mass that is not the pure product being weighed.
The excess reagent
Whatever is not the limiting reagent is in excess, and some of it is left over once the reaction stops. The amount consumed follows the same stoichiometric ratio as the product; subtracting that from what was originally available gives the leftover quantity — useful for working out how much of an expensive reactant to buy, or how much unreacted material needs to be separated from the product afterward.