# Explain why the mass of one mole of carbon-12 is exactly 12 g, whereas the molar mass of naturally occurring carbon is approximately 12.01 g mol⁻¹.

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

## Mark scheme (4 marks)

1. The mole is defined such that one mole of carbon-12 atoms has a mass of exactly 12 g, making it the reference standard for atomic mass.
2. Naturally occurring carbon is a mixture of isotopes, principally carbon-12 and carbon-13 (and trace carbon-14), with different masses.
3. The molar mass of naturally occurring carbon is a weighted average of the masses of its isotopes, taking into account their relative natural abundances.
4. Because carbon-13 is present in about 1.1% natural abundance and has a mass greater than 12 amu, the weighted average molar mass is slightly greater than 12 g mol⁻¹, giving approximately 12.01 g mol⁻¹.

## Key terms

- [mole](https://www.gradenine.co.uk/glossary/mole)
- [molar mass](https://www.gradenine.co.uk/glossary/molar-mass)
- [carbon-12](https://www.gradenine.co.uk/glossary/carbon-12)

## 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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