Limiting Reactants Explained
By the Periodixy Editorial Team · Last reviewed July 10, 2026
Mix reactants in anything other than their perfect recipe ratio and one of them runs out first — the limiting reactant. The moment it is gone, the reaction stops, no matter how much of everything else remains. Finding it is the key to predicting real yields.
The sandwich analogy: with 10 slices of bread and 3 fillings you can make 5 sandwiches only if fillings allow — with just 3 fillings, filling is limiting and 4 slices of bread sit unused.
The divide-by-coefficient method
- Balance the equation.
- Convert every given reactant amount to moles.
- Divide each reactant's moles by its coefficient — this gives “how many complete reaction runs” it can supply.
- The smallest result identifies the limiting reactant.
- Use the limiting reactant's moles for all further product calculations.
Fully worked example
Ammonia synthesis
N₂ + 3H₂ → 2NH₃. If 2.0 mol N₂ and 4.5 mol H₂ react, what is the limiting reactant and how much NH₃ forms?
- N₂: 2.0 ÷ 1 = 2.0 possible runs.
- H₂: 4.5 ÷ 3 = 1.5 possible runs ← smaller.
- H₂ is limiting. Use it: 4.5 mol H₂ × (2 NH₃ / 3 H₂) = 3.0 mol NH₃.
- Excess N₂ used: 1.5 runs × 1 = 1.5 mol → 0.5 mol N₂ left over.
Answer: H₂ limits; 3.0 mol NH₃ forms; 0.5 mol N₂ remains unreacted.
Theoretical yield and percent yield
The theoretical yield is the maximum product the limiting reactant allows — what a perfect reaction would deliver. Real experiments lose material to side reactions and transfers, giving an actual yield below the theoretical:
Percent yield
% yield = (actual yield ÷ theoretical yield) × 100
Percent yield
The reaction above theoretically gives 3.0 mol NH₃ (51.1 g), but the lab isolates 42.0 g. Find the percent yield.
- % yield = 42.0 ÷ 51.1 × 100
Answer: ≈ 82% — a typical, respectable lab result.
The Limiting Reactant Calculator runs the whole method — limiting reactant, theoretical yield and leftover excess — for any balanced equation.
Summary
- The limiting reactant runs out first and determines all product amounts.
- Method: moles ÷ coefficient for each reactant; smallest value = limiting.
- Theoretical yield comes from the limiting reactant; % yield = actual ÷ theoretical × 100.
- Never judge by raw amounts alone — coefficients decide.