Explain what is meant by the term 'mole' and describe how the molar mass of a substance is used to find the number of moles in a given mass of that substance.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
A mole is the amount of substance that contains the same number of particles as there are atoms in 12 g of carbon‑12 (6.02×10²³ particles). The molar mass is the mass of one mole of a substance and is expressed in g mol⁻¹. It is calculated by adding the relative atomic masses of all atoms in the chemical formula. The number of moles in a given mass is found using the relationship n = m ÷ M, where n is moles, m is mass (g) and M is molar mass (g mol⁻¹). For example, a larger molar mass means fewer moles for the same mass, because dividing the same mass by a larger number gives a smaller result.
Examiner tips
- Use the exact definition of a mole and Avogadro’s number; include units for molar mass; show the formula for n = m ÷ M; give a clear example linking larger molar mass to fewer moles; keep sentences concise and use correct terminology.
Common mistakes
- Writing the mole as 6.02×10²³ atoms instead of particles; forgetting to include units g mol⁻¹ for molar mass; misapplying the formula by multiplying instead of dividing mass by molar mass.
Mark scheme (5 marks)
- A mole is the amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (Avogadro's number / 6.02 × 10²³ particles)
- Molar mass is the mass of one mole of a substance / has units of g/mol
- The molar mass is found by adding together the relative atomic masses (Ar) of all atoms in the formula
- Number of moles = mass ÷ molar mass (correct relationship stated in words or symbols)
- Correct worked application: e.g. a larger molar mass means fewer moles for the same mass / dividing a given mass by a larger molar mass gives a smaller number of moles (or equivalent correct statement showing understanding of the relationship)
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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