# Explain why the empirical formula mass and the molar mass of a compound are not always equal, and outline how a chemist could use the molar mass of a compound, determined experimentally, to find its molecular formula from its empirical formula.

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

## Mark scheme (4 marks)

1. The empirical formula gives only the simplest whole-number ratio of atoms in a compound, not necessarily the actual number of atoms per molecule.
2. The molar mass of the molecular compound is a whole-number multiple (n) of the empirical formula mass.
3. The value of n is calculated by dividing the experimentally determined molar mass by the empirical formula mass.
4. The molecular formula is then obtained by multiplying all subscripts in the empirical formula by n.

## Key terms

- [empirical formula](https://www.gradenine.co.uk/glossary/empirical-formula)
- [molecular formula](https://www.gradenine.co.uk/glossary/molecular-formula)
- [molar mass](https://www.gradenine.co.uk/glossary/molar-mass)
- [empirical formula mass](https://www.gradenine.co.uk/glossary/empirical-formula-mass)

## 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)
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