# Explain how the empirical formula of a compound can be determined from its percentage composition by mass, and outline why the mole concept is essential in this process.

> IB DP Chemistry Standard Level (2023 syllabus) — S1.4 Counting particles by mass: The mole · Explain · 4 marks

## Mark scheme (4 marks)

1. Percentage by mass values are converted to an amount in moles by dividing each by the respective molar mass (atomic mass).
2. The mole ratios are obtained by dividing all mole values by the smallest mole value, giving a ratio of atoms.
3. These whole-number ratios give the simplest ratio of atoms of each element, which defines the empirical formula.
4. The mole concept is essential because it links mass (macroscopic, measurable) to the number of atoms (sub-microscopic), allowing mass ratios to be interpreted as atomic ratios.

## Key terms

- [empirical formula](https://www.gradenine.co.uk/glossary/empirical-formula)
- [percentage composition by mass](https://www.gradenine.co.uk/glossary/percentage-composition-by-mass)
- [mole](https://www.gradenine.co.uk/glossary/mole)

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-empirical-formula-of-46f0976c) · Published by Druglandscape Ltd.