Relative Formula Mass

Key Stage 4

Meaning

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

NB One mole is 6.02x10²³ molecules.

About Relative Formula Mass

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 Relative Formula Mass of O2 = 16x2 Relative Formula Mass of O2 = 32g	Relative Atomic Mass of Carbon = 12g Relative Atomic Mass of Oxygen = 16g No. Carbon atoms = 1 No. Oxygen atoms = 2 Relative Formula Mass of CO2 = 12x1 + 16x2 Relative Formula Mass of CO2 = 44g	Relative Atomic Mass of Carbon = 12g Relative Atomic Mass of Hydrogen = 1g Relative Atomic Mass of Oxygen = 16g No. Carbon atoms = 6 No. Carbon atoms = 12 No. Oxygen atoms = 6 Relative Formula Mass of C6H12O6 = 12x6 + 1x12 + 16x6 Relative Formula Mass of C6H12O6 = 180g