# 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

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

## 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

- [relative formula mass](https://www.gradenine.co.uk/glossary/relative-formula-mass)
- [mole](https://www.gradenine.co.uk/glossary/mole)

## Related

- [Revision notes for AQA A-Level Chemistry (7405)](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-how-a-student-could-use-04ee33c0) · Published by Druglandscape Ltd.