The reactant that is completely used up is called the limiting reactant…

The idea

The limiting reactant is the substance that runs out first and therefore controls how much product can form. Once it is used up, the reaction stops, even if other reactants remain. For example, if 0.10 mol of magnesium reacts with 0.20 mol of oxygen, the equation 2Mg + O₂ → 2MgO shows magnesium is limiting because 0.10 mol Mg needs only 0.05 mol O₂, leaving oxygen in excess. The amount of product is calculated from the limiting reactant using the mole ratio. Identifying the limiting reactant is essential in theoretical yield calculations and in predicting whether a reaction goes to completion, but it does not affect atom economy.

What earns the marks

• Define the limiting reactant as the reactant that is completely used up in a reaction.
• Explain that the limiting reactant determines the maximum amount of product that can form.
• Compare amounts in moles with the balanced equation ratio to identify which reactant is limiting.
• Use the limiting reactant's amount and the equation ratio to calculate the amount of product.
• Recognise that reaction stops when the limiting reactant is exhausted, leaving excess reactants unreacted.

Common mistakes and how to put them right

• Choosing the reactant with the smaller mass as limiting without converting to moles.

  Put it right by comparing moles using the equation ratio.

• Assuming the reactant present in the smallest mole amount is always limiting.

  Put it right by checking the stoichiometric ratio, since coefficients matter.

• Calculating product from the excess reactant.

  Put it right by using the limiting reactant for product calculations.

Exam tips

• Write the balanced equation and note the mole ratio before deciding which reactant is limiting.
• Show the comparison of required versus available moles for each reactant to justify your choice.
• Use the limiting reactant in subsequent product calculations and state this explicitly.