Limiting Reagent: How to Find It and Calculate Theoretical Yield
Find the limiting reagent in four steps, then calculate the theoretical yield, with a worked example.
The 4-step method
- 1. Balance the equation
- 2. Convert each reactant to moles
- 3. Divide moles by its coefficient
- 4. The smaller result is the limiting reagent
The limiting reagent runs out first and caps the product.
EXAMPLE
2 H₂ + O₂ → 2 H₂O. You have 4.0 mol H₂ and 3.0 mol O₂. Which is limiting?
- H₂: 4.0 ÷ 2 = 2.0
- O₂: 3.0 ÷ 1 = 3.0
- 2.0 is smaller, so H₂ is limiting
H₂ is limiting
Theoretical yield
How much H₂O forms from the 4.0 mol H₂?
- Always use the limiting reagent
- 4.0 mol H₂ × (2 mol H₂O / 2 mol H₂)
4.0 mol H₂O
Common mistake
The reactant with fewer moles is limiting
Compare moles divided by coefficient, not raw moles.
Convert grams to moles first. Comparing grams is a common mistake.
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