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Mendeleev · Chemistry pathway · level 2

Limiting reactant

The limiting reactant is consumed first according to stoichiometric proportions. It sets the maximum amount of product.

The essential idea

The limiting reactant is consumed first according to stoichiometric proportions. It sets the maximum amount of product.

Example

With 3 mol H₂ and 1 mol O₂ for 2 H₂ + O₂ → 2 H₂O, O₂ is limiting: it consumes 2 mol H₂ and leaves 1 mol H₂ in excess.

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Mini problem

Why isn't the reactant present in the smaller amount always limiting?

Show the solution

Because available amounts must be compared with the ratios required by the balanced equation.

Common mistake

Do not replace reasoning with a memorized rule. Always check geometry, units, coefficients or model assumptions.

Guided exercise

With 5 mol H₂ and 2 mol O₂, which reactant limits 2 H₂ + O₂ → 2 H₂O?

Show the detailed solution

2 mol O₂ require 4 mol H₂. Since 5 mol H₂ are available, O₂ is limiting and 1 mol H₂ remains in excess.

Avoid this

Never choose the limiting reactant by simply comparing the available mole counts.

Go further

Limiting reactantReview stoichiometry
Safer comparison

Divide amount by coefficient

The smallest n/coefficient value identifies the limiting reactant.

Key idea

Limiting reactant

Key idea

Limiting reactant

With 3 mol H₂ and 1 mol O₂ for 2 H₂ + O₂ → 2 H₂O, O₂ is limiting: it consumes 2 mol H₂ and leaves 1 mol H₂ in excess.

Guided example

Limiting reactant

With 3 mol H₂ and 1 mol O₂ for 2 H₂ + O₂ → 2 H₂O, O₂ is limiting: it consumes 2 mol H₂ and leaves 1 mol H₂ in excess.

Put it into practice

Limiting reactant

Choose a concrete case linked to Limiting reactant. Write down the known information, what you are trying to find, and the units. Then state the rule, model or trend you will use. If a calculation is involved, keep the units visible at every line; if it is conceptual, state what changes and what remains fixed.

Check your answer

A strong answer contains four things: the starting situation, the variable that changes, the direction or numerical result, and one check. The check can be a unit, an order of magnitude, a limiting case or a comparison with a familiar example.

Periodic table