State what is meant by the term 'relative molecular mass' and explain how the relative molecular mass of sulfuric acid, H₂SO₄, is obtained from the relative atomic masses of its constituent elements.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Relative molecular mass is the mass of a molecule compared to one‑twelfth of the mass of a carbon‑12 atom.
The relative molecular mass of a compound is found by adding the relative atomic masses of all atoms in the molecule.
For H₂SO₄ there are 2 H atoms, 1 S atom and 4 O atoms.
(2 × 1) + (1 × 32) + (4 × 16) = 2 + 32 + 64 = 98.
Hence the relative molecular mass of sulfuric acid is 98.
The relative molecular mass of a compound is found by adding the relative atomic masses of all atoms in the molecule.
For H₂SO₄ there are 2 H atoms, 1 S atom and 4 O atoms.
(2 × 1) + (1 × 32) + (4 × 16) = 2 + 32 + 64 = 98.
Hence the relative molecular mass of sulfuric acid is 98.
Examiner tips
- Use the exact definition of relative molecular mass; mention C‑12 = 12. Show the formula for the calculation and include the number of each atom. Give the final value 98.
- Use the correct atomic masses (H = 1, S = 32, O = 16).
Common mistakes
- Confusing relative atomic mass with atomic weight. Forgetting to multiply each atomic mass by the correct number of atoms. Writing the final mass as 98 g mol⁻¹ instead of simply 98.
Mark scheme (4 marks)
- Relative molecular mass is the mass of a molecule compared to one-twelfth of the mass of a carbon-12 atom (or: the average mass of a molecule on a scale where carbon-12 = 12)
- Relative molecular mass is found by adding together the relative atomic masses of all atoms present in the molecule
- Correct identification and use of the number of each type of atom in H₂SO₄: 2 hydrogen atoms, 1 sulfur atom, 4 oxygen atoms
- Correct calculation shown: (2 × 1) + (1 × 32) + (4 × 16) = 98, giving a relative molecular mass of 98
Key terms in this question
relative molecular mass · relative atomic mass
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "State and Explain" question →
- Decode the mark scheme abbreviations →
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