# Explain why the molar mass of chlorine gas (Cl₂) is approximately twice the relative atomic mass of chlorine listed on the periodic table, and state the unit of molar mass.

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

> Chlorine exists as a diatomic molecule, Cl₂, under standard conditions. The relative atomic mass of chlorine is approximately 35.5.

## Mark scheme (4 marks)

1. The relative atomic mass of chlorine (35.5) is a weighted average of the masses of its naturally occurring isotopes (Cl-35 and Cl-37) relative to 1/12 the mass of carbon-12.
2. Chlorine exists as a diatomic molecule (Cl₂), so one mole of chlorine gas contains two moles of chlorine atoms.
3. The molar mass of Cl₂ is therefore approximately 71 (2 × 35.5).
4. The unit of molar mass is g mol⁻¹ (grams per mole).

## Key terms

- [molar mass](https://www.gradenine.co.uk/glossary/molar-mass)
- [relative atomic mass](https://www.gradenine.co.uk/glossary/relative-atomic-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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