This topic is taught at both GCSE and at A level.
Definition: The empirical formula is the simplest whole number ratio of atoms of each element in a compound.
Hydrogen peroxide has a molecular formula of H2O2. Whilst its Empirical formula is HO. Benzene has a molecular formula of C6H6. Whilst its Empirical formula is CH. An empirical may not actually exist in real life, its just a way of cancelling the number of atoms on a piece of paper.
So the questions will either ask you to cancel an existing formula like above or they will give you percentages or masses of elements in a compound.
Four step process to dealing with an empirical formula question:
1. Put the elements at the top of the page
2. Put the masses or % under each element
3. Divide by atomic mass
4. Then divide by smallest.
However, there are a few sneaky questions and tricks that they can use. I will show you them in videos. As the videos progress they show progressively harder questions.
1. Calculate the empirical formula of a hydrocarbon that contains 85.7% carbon and 14.3% hydrogen.