Can there be more than two reactants?

Yes. Apply the exact same procedure: calculate the theoretical yield of the chosen product for every reactant. The one producing the lowest yield is limiting.

What if the reactants produce the exact same amount of product?

Then the reactants are present in exactly stoichiometric proportions. Neither is limiting, as both are completely consumed simultaneously.

Do I have to calculate the moles of product?

Alternatively, you can divide the initial moles of each reactant by its stoichiometric coefficient from the balanced equation. The reactant with the smallest resulting ratio is the limiting reactant.

How to find the limiting reactant

The limiting reactant is the substance that is completely consumed first in a chemical reaction, determining the maximum amount of product that can be formed. It applies to any reaction where reactants are mixed in non-stoichiometric ratios.

To find it, convert all reactant quantities to moles, then calculate the moles of a chosen product each would produce based on the balanced chemical equation. The reactant yielding the smallest amount of product is the limiting reactant.

The setup

You need a balanced chemical equation and the initial quantities (mass, volume, or moles) of all reactants. Select one product to serve as the basis for comparison.

The steps

Balance the chemical equation.
Convert the given initial quantity of each reactant into moles.
For each reactant, multiply its initial moles by the stoichiometric ratio (moles of chosen product / moles of reactant) to find the theoretical moles of product it could form.
Compare the calculated moles of product. The reactant that produces the smallest value is the limiting reactant.

Checking the result

Verify your stoichiometric ratios and molar masses. To check for consistency, calculate the mass of the excess reactant(s) remaining after the reaction. If any remaining mass is negative, the limiting reactant was identified incorrectly.

Common errors

Comparing the initial moles or masses of reactants directly without accounting for the stoichiometric coefficients. Failing to balance the chemical equation before determining molar ratios.

Worked example

10.0 g of aluminum ( $Al$ ) reacts with 35.0 g of chlorine gas ( $Cl_2$ ) to form aluminum chloride ( $AlCl_3$ ). Find the limiting reactant.

Balanced equation: $2Al + 3Cl_2 \rightarrow 2AlCl_3$

Molar masses: $Al = 26.98$ g/mol, $Cl_2 = 70.90$ g/mol.

Calculate initial moles: Moles of $Al = \frac{10.0}{26.98} = 0.371$ mol Moles of $Cl_2 = \frac{35.0}{70.90} = 0.494$ mol

Calculate moles of $AlCl_3$ formed by each reactant: From $Al$ : $0.371 ext{ mol } Al imes \frac{2 ext{ mol } AlCl_3}{2 ext{ mol } Al} = 0.371 ext{ mol } AlCl_3$ From $Cl_2$ : $0.494 ext{ mol } Cl_2 imes \frac{2 ext{ mol } AlCl_3}{3 ext{ mol } Cl_2} = 0.329 ext{ mol } AlCl_3$

Since $Cl_2$ yields a smaller amount of product ( $0.329$ mol < $0.371$ mol), $Cl_2$ is the limiting reactant.

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References: OpenStax Chemistry 2e: Chapter 4.2 Stoichiometry · Zumdahl Chemical Principles: Chapter 3