Limiting Reagent Calculator
Compare two reactants and their coefficients to find which one is the limiting reagent and which is in excess.
How it works
ratio = moles ÷ coefficient · the reactant with the smaller ratio is limiting · excess left = excess moles − (limiting ratio × excess coefficient)
The limiting reagent is the reactant that runs out first and therefore caps how much product forms. Divide each reactant's moles by its balanced-equation coefficient; the reactant with the smaller quotient is limiting, and the other is in excess. Multiplying the limiting ratio by the excess reactant's coefficient tells you how much of the excess is actually used, so the remainder is left over.
Worked example
For A + 3B → products with 2 mol A and 5 mol B: A gives 2 ÷ 1 = 2, B gives 5 ÷ 3 = 1.67. B has the smaller ratio, so B is limiting; A used is 1.67 × 1 = 1.67 mol, leaving 2 − 1.67 = 0.33 mol of A.
Sources: Limiting reagent — Wikipedia
Frequently asked questions
Why divide moles by the coefficient?
Because a reactant with a bigger coefficient is consumed faster per unit of reaction. Dividing moles by coefficient puts both reactants on the same "reaction extent" scale so they can be compared fairly.
What is the excess reagent?
The reactant that is left over after the limiting one is used up. Some of it reacts, but not all — the leftover amount is shown as excess remaining.
How does this feed into yield?
The limiting reagent sets the moles of product possible, which is the starting point for theoretical and percent yield calculations.
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Educational estimate. Check the formula, units and significant figures for your exact case.