# Relative Formula Mass

## Key Stage 4

### Meaning

The relative formula mass (Mr) of a molecule is the mass of one mole of molecules.

NB One mole is 6.02x1023 molecules.

The relative formula mass is used to find the mass of a molecule needed to perform a complete reaction with another chemical.

### Calculating Relative Formula Mass

Relative formula mass can be calculated from the chemical formula of the molecule and the relative atomic mass of the elements.
 Find the relative formula mass of O2. Find the relative formula mass of CO2 Find the relative formula mass of C6H12O6 Relative Atomic Mass of Oxygen = 16g No. Oxygen atoms = 2 Mr of O2 = 16x2 Mr = 32g Relative Atomic Mass of Carbon = 12g Relative Atomic Mass of Oxygen = 16g No. Carbon atoms = 1 No. Oxygen atoms = 2 Mr of CO2 = 12x1 + 16x2 Mr = 44g Relative Atomic Mass of Carbon = 12g Relative Atomic Mass of Oxygen = 16g No. Carbon atoms = 6 No. Carbon atoms = 12 No. Oxygen atoms = 6 Mr of C6H12O6 = 12x6 + 1x12 + 16x6 Mr = 180g

### References

#### AQA

Relative formula mass (Mr), pages 64-5, GCSE Chemistry, Hodder, AQA
Relative formula mass, M, page 123, GCSE Combined Science; The Revision Guide, CGP, AQA
Relative formula mass, M, pages 41-48, GCSE Chemistry; The Revision Guide, CGP, AQA
Relative formula mass, pages 100-1, 106-8, 115-7, 120-3, 125, 167, GCSE Chemistry; Student Book, Collins, AQA
Relative formula mass, pages 104-108, 114-117, GCSE Combined Science Trilogy; Chemistry, CGP, AQA
Relative formula mass, pages 110-114, 120-124, GCSE Chemistry, CGP, AQA
Relative formula mass, pages 177-8, GCSE Combined Science Trilogy 1, Hodder, AQA
Relative formula mass, pages 62-63, GCSE Chemistry; Third Edition, Oxford University Press, AQA

#### Edexcel

Relative formula mass, page 72, GCSE Chemistry, Pearson, Edexcel
Relative formula masses, pages 27, 28, GCSE Chemistry; The Revision Guide, CGP, Edexcel
Relative formula masses, pages 71, 85, GCSE Chemistry, CGP, Edexcel
Relative formula masses, pages 90, 91, GCSE Combined Science; The Revision Guide, CGP, Edexcel

#### OCR

Relative formula mass (Mr), page 104, Gateway GCSE Combined Science; The Revision Guide, CGP, OCR
Relative formula mass (Mr), pages 31, 38, Gateway GCSE Chemistry; The Revision Guide, CGP, OCR
Relative formula mass, pages 37, 39, Gateway GCSE Chemistry, Oxford, OCR