Percentage composition of compounds

Having worked out the Mr of a compound, the percentage of that compound that is made from a particular element can now be worked out. This is done using the Mr of the compound and the Ar of the element we are interested in.

% mass = (Total Ar of the element ÷ Mr of the compound) × 100

Here are two examples.

Question

Find the percentage of calcium in calcium carbonate, CaCO3.

Step one – Work out the Mr of the compound.

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

Step two – Work out the total Ar 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 Ar of the element by the Mr of the compound, then multiply by 100.

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

Question

Find the percentage of iron in iron(III) oxide, Fe2O3.

Step one – Work out the Mr of the compound.

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

Step two – Work out the total Ar 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 Ar of the element by the Mr of the compound, then multiply by 100.

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