# Explain how the law of conservation of mass is consistent with the concept of mole ratios in a balanced chemical equation, and outline how atom economy differs from percentage yield as a measure of the efficiency of a chemical reaction.

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

## Mark scheme (4 marks)

1. The law of conservation of mass states that mass (or atoms) is neither created nor destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products.
2. Mole ratios in a balanced equation reflect the fixed proportions in which atoms are rearranged, ensuring that the number of atoms of each element is the same on both sides, which is consistent with conservation of mass.
3. Atom economy is defined as the molar mass of the desired product divided by the total molar mass of all products (multiplied by 100%), and is a theoretical value intrinsic to the reaction pathway chosen.
4. Percentage yield compares the actual experimental amount of product obtained to the theoretical maximum calculated from stoichiometry, so it reflects real experimental losses, whereas atom economy reflects how wasteful the reaction is in principle regardless of experimental conditions.

## Key terms

- [law of conservation of mass](https://www.gradenine.co.uk/glossary/law-of-conservation-of-mass)
- [mole ratio](https://www.gradenine.co.uk/glossary/mole-ratio)
- [atom economy](https://www.gradenine.co.uk/glossary/atom-economy)
- [percentage yield](https://www.gradenine.co.uk/glossary/percentage-yield)

## Related

- [Revision notes for IB DP Chemistry Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-law-of-conservation-57daa52b) · Published by Druglandscape Ltd.