Reaction information is shown using word and symbol equations. Mass is conserved in chemical reactions. We can predict the masses of products and reactants involved in chemical reactions as no atoms are made or destroyed in chemical reactions. Relative atomic masses can be used to find the relative formula mass of a compound.

Having worked out the *M*_{r} 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 *M*_{r} of the compound and the *A*_{r} of the element we are interested in.

% mass = (Total *A*_{r} of the element ÷ *M*_{r} of the compound) × 100

Here are two examples.

- Question
Find the percentage of calcium in calcium carbonate, CaCO

_{3}.**Step one**– Work out the*M*_{r}of the compound.*M*_{r}of calcium carbonate, CaCO_{3}= 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 CaCO

_{3}, 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 CaCO

_{3}= (40 ÷ 100) × 100 = 40%

- Question
Find the percentage of iron in iron(III) oxide, Fe

_{2}O_{3}.**Step one**– Work out the*M*_{r}of the compound.*M*_{r}of iron(III) oxide, Fe_{2}O_{3}= (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 Fe

_{2}O_{3}, 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 Fe

_{2}O_{3}= (112 ÷ 160) × 100 = 70%