Limiting Reagent
Coefficients from your balanced equation
Compares product makeable from each reactant; the smaller wins. Methodology & sources →
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Convert each reactant to moles, then use the mole ratio to find how much product each could make alone. The reactant that makes the least is the limiting reagent, and that amount is the theoretical yield.
Read the full method in Limiting Reagents Explained, or do a single-substance conversion with the Stoichiometry Calculator.
Worked Example — Finding the Limiting Reagent and Theoretical Yield
Question: N₂(g) + 3 H₂(g) → 2 NH₃(g). If 28.0 g of N₂ and 6.00 g of H₂ are mixed, which is limiting, and what is the theoretical yield of NH₃?
Step 1 — convert both masses to moles: n(N₂) = 28.0 ÷ 28.02 = 0.999 mol. n(H₂) = 6.00 ÷ 2.016 = 2.976 mol
Step 2 — calculate the maximum NH₃ each reactant could produce alone:
from N₂: 0.999 × (2/1) = 1.999 mol NH₃
from H₂: 2.976 × (2/3) = 1.984 mol NH₃
Step 3 — the smaller value wins: H₂ produces less NH₃, so H₂ is the limiting reagent.
Step 4 — theoretical yield: m(NH₃) = 1.984 × 17.03 = 33.8 g
Answer: H₂ is limiting; theoretical yield = 33.8 g NH₃.
Common Mistakes
- Comparing grams directly. Whichever reactant has the smaller mass isn't necessarily limiting — the comparison must be done in moles, accounting for the reaction's mole ratio.
- Comparing moles directly without the ratio. Even after converting to moles, the reactant with fewer moles isn't automatically limiting if the equation needs more of it — here H₂ has more moles than N₂ but is still limiting, because 3 mol H₂ is needed per 1 mol N₂.
- Percent yield uses theoretical, not given, mass. % yield = (actual yield ÷ theoretical yield) × 100 — the theoretical yield must come from the limiting-reagent calculation, not from either starting mass.
Frequently Asked Questions
Convert each reactant to moles, find how much product each could make using the mole ratio, and the one that makes the least is limiting.
Percent yield = (actual yield ÷ theoretical yield) × 100%. Theoretical yield comes from the limiting reagent.
Convert every reactant's mass to moles, then calculate how much product each one could form on its own using the balanced equation's mole ratios. The reactant that produces the smallest amount of product is the limiting reagent — it runs out first and determines the maximum possible yield.
Theoretical yield is the maximum product possible, calculated from the limiting reagent assuming the reaction goes to completion with no losses. Actual yield is what you actually measure in the lab — always equal to or less than theoretical, due to side reactions, incomplete reactions, or losses during purification. Percent yield = actual ÷ theoretical × 100.