The relative formula mass (Mr) is the total of all the relative atomic masses (Ar) of the atoms in a compound. It tells us the mass of one mole of a substance in grams.

What does Mr mean?

• Mr stands for relative formula mass
• It is used for compounds made of two or more elements
• It is found by adding up the Ar (relative atomic masses) of all atoms in the formula
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How to calculate Mr

1. Write the formula of the compound
2. Find the Ar (relative atomic mass) of each element – from the periodic table
3. Multiply by how many atoms of each element
4. Add the total together

Examples

Example 1: H₂O

• H = 1, O = 16
• (2 × 1) + (1 × 16) = 18

Mr of H₂O = 18

Example 2: CO₂

• C = 12, O = 16
• (1 × 12) + (2 × 16) = 44

Mr of CO₂ = 44

Example 3: Ca(OH)₂

• Ca = 40, O = 16, H = 1
• 40 + (2 × 16) + (2 × 1) = 74

Mr of Ca(OH)₂ = 74

💡 Brackets mean you multiply everything inside by the subscript outside.

Why is Mr important?

• It helps us calculate how much of a substance is involved in a reaction
• Mr is used in mole calculations (covered in later lessons)
• It links mass, moles, and formulas