# Relative formula mass

The sum of the relative atomic masses of all the atoms shown in a formula. For a molecule it is also called relative molecular mass (Mr).

Chemistry (school) · Part 05: Moles and calculations · Card 2 of 9 · Last updated 2026-10-19

## Analogy

Like adding up the weights of every brick in a model.

## Example

**Mr of CO2**

C = 12
O = 16, two of them = 32
Mr = 12 + 32 = 44

## In practice

You need it to work out how many moles of a compound you have.

## Practice

### Flashcards

- **What is the formula linking moles, mass and Mr?** Moles = mass / Mr.
- **What is the Mr of water, H2O?** 18 (2 x 1 + 16).

### Interview questions

**Calculate the mass of 0.5 mol of calcium carbonate, CaCO3. (Ar: Ca = 40, C = 12, O = 16)**

- Mr = 40 + 12 + 3 x 16 = 100
- Mass = moles x Mr
- 0.5 x 100 = 50 g

## Questions people ask

### How do you calculate Mr for a compound with brackets?

Work out the mass of the group in the brackets, then multiply by the number outside. For calcium hydroxide, Ca(OH)2, the OH group is 16 + 1 = 17. Two of them make 34. Add calcium at 40 to get an Mr of 74.

### Does relative formula mass have units?

No. It is a ratio compared with carbon-12, so the units cancel out. When you use it to find the mass of one mole, you add grams: Mr of 18 means 18 g per mole. In a calculation the number is used, with units added only at the end.

## Related concepts

- [Moles from mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-5)
- [Relative atomic mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-1)
- [Mole](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-3)
- [Conservation of mass](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-4/chemistry-school-part-4-concept-2)

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