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Step one – Work out the M_r of the compound.

M_r of calcium carbonate, CaCO₃ = 40 + 12 + (3 × 16) = 100

Step two – Work out the total A_r of all of the atoms of the element you are interested in.

There is only one atom of calcium in CaCO₃, so the total mass of all the calcium atoms = 1 × 40 = 40

Step three – Divide the total A_r of the element by the M_r of the compound, then multiply by 100.

The percentage of calcium in CaCO₃ = (40 ÷ 100) × 100 = 40%

Step one – Work out the M_r of the compound.

M_r of iron(III) oxide, Fe₂O₃ = (2 × 56) + (3 × 16) = 160

Step two – Work out the total A_r of all of the atoms of the element you are interested in.

There are two atoms of iron in Fe₂O₃, so the total mass of all the iron atoms = 2 × 56 = 112

Step three – Divide the total A_r of the element by the M_r of the compound, then multiply by 100.

The percentage of iron in Fe₂O₃ = (112 ÷ 160) × 100 = 70%