# Explain how the concept of a limiting reagent determines the theoretical yield of a product in a chemical reaction, and outline how a chemist would identify which reagent is limiting when given the masses of two reactants and a balanced equation.

> IB DP Chemistry Standard Level (2023 syllabus) — R2.1 How much? The amount of chemical change · Explain · 4 marks

## Mark scheme (4 marks)

1. The limiting reagent is the reactant that is completely consumed first / is present in the smaller stoichiometric amount, stopping the reaction.
2. The theoretical yield is determined solely by the amount (in moles) of the limiting reagent, using the stoichiometric mole ratio from the balanced equation.
3. To identify the limiting reagent, convert the mass of each reactant to moles by dividing by its molar mass.
4. Divide the moles of each reactant by its stoichiometric coefficient from the balanced equation; the reactant with the smaller resulting value is the limiting reagent.

## Key terms

- [limiting reagent](https://www.gradenine.co.uk/glossary/limiting-reagent)
- [theoretical yield](https://www.gradenine.co.uk/glossary/theoretical-yield)

## Related

- [Revision notes for IB DP Chemistry Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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