Calculating Relative Formula Mass

Calculating Relative Formula Mass

The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of all the atoms in its formula.

Working it out

For water, H₂O: hydrogen has a relative atomic mass of 1, oxygen is 16. There are 2 hydrogens and 1 oxygen, so Mr = (2 × 1) + 16 = 18.

Why it matters

Relative formula mass lets chemists work out how much of each substance is needed or produced in a reaction, which is the basis for all quantitative chemistry calculations.

Practice

Calculate the relative formula mass of CO₂ (carbon = 12, oxygen = 16).