Empirical formulae

through analysis of substance

If we know the percentage composition of a substance we can calculate its empirical formula.
Lets try an example to see how.
A substance containing copper and oxygen was analysed and its composition, by mass, was found to be 80% copper and 20% oxygen. Find its empirical formula.
Atomic mass of copper = 63.5, oxygen =16
Step 1 Assume we have 100 grams of the substance. The percentage of each element can now be converted to mass.
So we have 80 grams of copper and 20 grams of oxygen.

Step 2. Convert the mass of each element into mole.
mole of copper = mass/Atomic mass = 80/63.5 = 1.25 mole
mole of oxygen = mass/Atomic mass = 20/16 = 1.25 mole

Step 3 Find the simplest mole ratio of each element.

Cu : O
1.25 : 1.25
1 : 1

Step 4 The empirical formula is CuO

Worksheet Solutions