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.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The mole is defined so that one mole of carbon‑12 atoms has a mass of exactly 12 g, providing a fixed reference for atomic masses.
Naturally occurring carbon is a mixture of isotopes – mainly carbon‑12 and carbon‑13 (with trace carbon‑14). Each isotope has a different mass.
The molar mass of natural carbon is therefore the weighted average of the isotope masses, weighted by their natural abundances.
Because about 1.1 % of natural carbon is carbon‑13, which is heavier than carbon‑12, the weighted average is slightly above 12 g mol⁻¹, giving ≈12.01 g mol⁻¹.
Naturally occurring carbon is a mixture of isotopes – mainly carbon‑12 and carbon‑13 (with trace carbon‑14). Each isotope has a different mass.
The molar mass of natural carbon is therefore the weighted average of the isotope masses, weighted by their natural abundances.
Because about 1.1 % of natural carbon is carbon‑13, which is heavier than carbon‑12, the weighted average is slightly above 12 g mol⁻¹, giving ≈12.01 g mol⁻¹.
Examiner tips
- Define the mole first, then explain isotope mixture and weighted average; use exact numbers (1.1 %) to justify 12.01 g
Common mistakes
- Confusing the definition of the mole with the molar mass of a specific isotope; forgetting to mention isotope weighting
Mark scheme (4 marks)
- 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.
- Naturally occurring carbon is a mixture of isotopes, principally carbon-12 and carbon-13 (and trace carbon-14), with different masses.
- The molar mass of naturally occurring carbon is a weighted average of the masses of its isotopes, taking into account their relative natural abundances.
- 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 in this question
mole · molar mass · carbon-12
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