Describe how a student could use the relative formula mass of a substance and a known number of moles to calculate the mass of that substance needed for an experiment. Use the compound calcium carbonate (CaCO₃) as your example throughout your answer.

AQA A-Level Chemistry (7405) — 3.1.2 Amount of substance · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Relative atomic masses: Ca = 40, C = 12, O = 16

Model answer (5 marks)

1. Add the relative atomic masses of the atoms in CaCO₃: Ca 40 + C 12 + 3×O 16 = 40 + 12 + 48 = 100.
2. The relative formula mass (Mr) of CaCO₃ is therefore 100.
3. The relationship between mass, moles and Mr is mass = moles × Mr (or moles = mass ÷ Mr).
4. The molar mass of CaCO₃ in g mol⁻¹ is numerically equal to its Mr, so 100 g mol⁻¹.
5. For example, to prepare 0.50 mol of CaCO₃, the required mass is 0.50 mol × 100 g mol⁻¹ = 50 g.

Examiner tips

  • Use the exact formula mass 100; show the addition. State the equation mass = moles × Mr. Mention that Mr = molar mass in g mol⁻¹. Give a numerical example with the calculation shown.

Common mistakes

  • Using 48 for the O contribution instead of 3×16. Confusing Mr with molar mass units. Omitting the multiplication step in the example calculation.

Mark scheme (5 marks)

  1. Add up the relative atomic masses of all atoms in the formula to find the relative formula mass (Mr)
  2. Correct calculation of Mr of CaCO₃ = 100 (40 + 12 + 48)
  3. State the relationship: mass = moles × Mr (or moles = mass ÷ Mr)
  4. Explain that the molar mass in grams per mole is numerically equal to the Mr
  5. Apply the method to give a correct example mass for a stated number of moles of CaCO₃

Key terms in this question

relative formula mass · mole

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