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Limiting Reactant Problems: Step-by-Step

Identify which reagent runs out first and calculate theoretical product amount.

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What is the limiting reactant?

The limiting reactant is the substance consumed first when a reaction proceeds according to its balanced equation. The amount available determines the maximum theoretical amount of product. Other reactants remain in excess.

Illustrated Limiting Reactant Problems: Step-by-Step study overview
IntoChemistry visual guide. Follow the worked examples below for the complete reasoning.

The reliable four-step method

  1. Balance the reaction before doing any amount calculations.
  2. Convert mass or particles to moles as necessary.
  3. Compute available moles / coefficient for every reactant, then select the smallest positive value.
  4. Multiply that smallest value by the product coefficient. Convert product moles to grams if asked.

See the reaction-extent comparison

Comparison of 3.00 mol H2 divided by coefficient 2 versus 1.00 mol O2 divided by coefficient 1, giving O2 as limiting.
In this example oxygen sets a reaction extent of 1.00, so at most 2.00 mol H₂O can form.

Worked problems

Example 1

Water from hydrogen and oxygen

  1. Balanced: 2 H₂ + O₂ → 2 H₂O.
  2. Available: 3.00 mol H₂ and 1.00 mol O₂.
  3. Extents: 3.00 / 2 = 1.50 for H₂; 1.00 / 1 = 1.00 for O₂.
  4. O₂ is limiting; H₂O = 1.00 × 2 = 2.00 mol.
  5. H₂ consumed = 2.00 mol; 1.00 mol H₂ remains.

O₂ limits the reaction; 2.00 mol water theoretically forms.

Example 2

Ammonia production

  1. N₂ + 3 H₂ → 2 NH₃.
  2. Available: 1.50 mol N₂ and 3.00 mol H₂.
  3. Extents: 1.50 / 1 = 1.50; 3.00 / 3 = 1.00.
  4. H₂ is limiting. Maximum NH₃ = 1.00 × 2 = 2.00 mol.
  5. Using 17.03 g/mol for NH₃, theoretical mass is 34.1 g.

H₂ limits the reaction; 2.00 mol or approximately 34.1 g NH₃.

Common misconceptions

  • The reactant with fewer moles is not necessarily limiting.
  • The theoretical yield is a model maximum; an actual yield may be smaller.
  • The reactants must be expressed in comparable mole units before using ratios.
  • When reaction extents are equal, neither reactant is in excess under the ideal stoichiometric model.

Quick check

For N₂ + 3 H₂ → 2 NH₃, which reactant limits when you have 2.00 mol N₂ and 3.00 mol H₂?

H₂: ratios are 2.00/1 = 2.00 for N₂ versus 3.00/3 = 1.00 for H₂. Maximum NH₃ is 2.00 mol.

Interactive Limiting Reactant Problems: Step-by-Step Practice

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Limiting reactant calculator practice

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