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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Chlorine exists as a diatomic molecule, Cl₂, under standard conditions. The relative atomic mass of chlorine is approximately 35.5.
Model answer (4 marks)
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.
Chlorine exists as a diatomic molecule (Cl₂), so one mole of chlorine gas contains two moles of chlorine atoms.
Therefore the molar mass of Cl₂ is approximately 71 g mol⁻¹ (2 × 35.5).
The unit of molar mass is g mol⁻¹ (grams per mole).
Chlorine exists as a diatomic molecule (Cl₂), so one mole of chlorine gas contains two moles of chlorine atoms.
Therefore the molar mass of Cl₂ is approximately 71 g mol⁻¹ (2 × 35.5).
The unit of molar mass is g mol⁻¹ (grams per mole).
Examiner tips
- Use the word ‘therefore’ to link the facts; show the multiplication 2×35.5; write the unit exactly as g mol⁻¹.
- Mention the isotopic weighting only briefly – it is enough to state the 35.5 figure is a weighted average.
Common mistakes
- Writing the unit as mol⁻¹ or g mol instead of g mol⁻¹; "g per mole" is the correct form.
- Forgetting to double the atomic mass because of the diatomic nature of Cl₂; students sometimes give 35.5 instead of 71.
Mark scheme (4 marks)
- 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.
- Chlorine exists as a diatomic molecule (Cl₂), so one mole of chlorine gas contains two moles of chlorine atoms.
- The molar mass of Cl₂ is therefore approximately 71 (2 × 35.5).
- The unit of molar mass is g mol⁻¹ (grams per mole).
Key terms in this question
molar mass · relative atomic mass
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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