Real reactions almost never start with reactants in exactly the right proportion. One of them always runs out before the other, and that reactant — the limiting reactant — is the one that actually determines how much product you get. This calculator compares two reactants' available moles against the balanced-equation coefficients to find which one limits the reaction, then reports the theoretical yield and how much of the other reactant is left over.
Why comparing moles alone isn't enough
It's tempting to assume whichever reactant you have less of (in grams, or even in moles) is automatically the limiting one, but that ignores the balanced equation. A reaction that needs three moles of one reactant for every mole of another can easily run out of the "three moles" reactant first even when you have more of it sitting on the bench. Dividing each reactant's available moles by its own coefficient — ratio = moles ÷ coefficient — puts both reactants on the same footing, and the smaller ratio identifies the reactant that runs out first.
From limiting reactant to theoretical yield
Once you know which reactant limits the reaction, the product calculation is direct: take the limiting reactant's moles, multiply by the mole ratio between it and the product from the balanced equation, then convert to grams using the product's molar mass. Critically, the excess reactant's amount never enters this calculation — having more of it sitting around doesn't create more product once the limiting reactant is gone.
What's left over
The excess reactant doesn't fully react. Only enough of it reacts to match the limiting reactant mole-for-mole (via the coefficient ratio); everything beyond that stays in the flask unreacted. Subtracting the moles that actually reacted from the moles you started with — then converting to grams — tells you exactly how much of the excess reactant remains, which matters for cost, safety, and cleanup in a real lab.
The scope of this calculator
This version handles exactly two reactants and one product, which covers the large majority of textbook and intro-lab limiting-reagent problems. Reactions with three or more reactants, or multiple simultaneous products (like a combustion reaction that produces both CO₂ and H₂O), need the comparison extended reactant-by-reactant or tracked with a separate calculation per product — this tool is scoped to the two-reactant case on purpose, to keep the inputs simple.