The limiting reagent is whichever reactant runs out first — and once it's gone, the reaction stops, no matter how much of the other reactant is left over. Finding it is one of the most common sources of chemistry homework help requests, mostly because students try to guess instead of using a reliable method.
The step-by-step method
- Balance the chemical equation if it isn't already.
- Convert both given reactant amounts to moles.
- Divide each reactant's moles by its coefficient in the balanced equation.
- Whichever reactant gives the smaller result is the limiting reactant. The other is in excess.
- Use only the limiting reactant's moles to calculate the amount of product formed — never the excess reactant.
Worked example
4 mol of H₂ react with 2 mol of O₂ in the reaction 2H₂ + O₂ → 2H₂O. Which is the limiting reactant?
H₂: 4 mol ÷ coefficient 2 = 2
O₂: 2 mol ÷ coefficient 1 = 2
Both give the same result (2), meaning they're in exact stoichiometric proportion — neither is in excess, so both are fully consumed. If instead O₂ had been 3 mol, O₂ ÷ 1 = 3, which is larger than H₂'s 2 — making H₂ the limiting reactant.
Common mistakes
Comparing raw mole amounts instead of dividing by the coefficient. Having more moles of a reactant doesn't automatically mean it's in excess — you have to account for the ratio the equation actually requires.
Using the excess reactant's moles to calculate product. Once you've identified the limiting reactant, every further calculation in the problem should be based on its moles, not the reactant left over.
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Get a free quoteNeed more help with college chemistry? This stoichiometry limiting reactant method works for any pair of reactants once the equation is balanced.