How the limiting reactant is found
- Convert every reactant amount to moles.
- Divide each by its coefficient — this counts how many complete “reaction runs” it can supply.
- The smallest value wins: that reactant is limiting and controls all product amounts.
Worked example
2.0 mol N₂ and 4.5 mol H₂ react via N₂ + 3H₂ → 2NH₃. Which limits, and how much NH₃ forms?
- N₂: 2.0 ÷ 1 = 2.0 runs • H₂: 4.5 ÷ 3 = 1.5 runs ← smaller
- H₂ is limiting despite being the larger amount — coefficients matter.
- NH₃ = 1.5 runs × 2 = 3.0 mol.
Answer: H₂ limits; 3.0 mol (≈ 51 g) of NH₃ forms; 0.5 mol of N₂ is left over.
⚠ Common mistake: The reactant with fewer grams or fewer moles is NOT automatically limiting — always divide by the coefficient first.
Theoretical vs actual yield
The theoretical yield is the ceiling — what forms if every limiting-reactant molecule reacts perfectly. Real experiments deliver less (side reactions, transfer losses), and the ratio actual ÷ theoretical × 100 is the percent yield. A calculated percent yield above 100% signals an error or an impure product.