Explain what is meant by the term 'amount of substance' and outline how the mole concept allows chemists to relate the mass of a sample of an element to the number of atoms it contains.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Amount of substance is a base SI quantity (symbol n) that measures the number of specified particles (atoms, molecules, ions, etc.) in a sample.
One mole contains Avogadro's constant (Nₐ ≈ 6.02 × 10²³) particles / elementary entities.
The molar mass of an element (in g mol⁻¹) is numerically equal to its relative atomic mass (Ar), linking mass to amount.
The number of atoms in any sample can therefore be found by dividing the mass of the sample by the molar mass to obtain n (in mol), then multiplying by Avogadro's constant (N = n × Nₐ).
One mole contains Avogadro's constant (Nₐ ≈ 6.02 × 10²³) particles / elementary entities.
The molar mass of an element (in g mol⁻¹) is numerically equal to its relative atomic mass (Ar), linking mass to amount.
The number of atoms in any sample can therefore be found by dividing the mass of the sample by the molar mass to obtain n (in mol), then multiplying by Avogadro's constant (N = n × Nₐ).
Examiner tips
- Use the exact definition of amount of substance and the symbol n. Mention Avogadro's constant and its numerical value. State the relationship between molar mass and relative atomic mass. Show the calculation steps: n = m / M, then N = n × Nₐ.
Common mistakes
- Confusing amount of substance with mass or moles. Using the wrong symbol (e.g., m instead of n). Omitting Avogadro's constant or giving an incorrect value. Failing to show the two‑step calculation from mass to atoms.
Mark scheme (4 marks)
- Amount of substance is a base SI quantity (symbol n) that measures the number of specified particles (atoms, molecules, ions, etc.) in a sample.
- One mole contains Avogadro's constant (Nₐ ≈ 6.02 × 10²³) particles / elementary entities.
- The molar mass of an element (in g mol⁻¹) is numerically equal to its relative atomic mass (Ar), linking mass to amount.
- The number of atoms in any sample can therefore be found by dividing the mass of the sample by the molar mass to obtain n (in mol), then multiplying by Avogadro's constant (N = n × Nₐ).
Key terms in this question
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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