Empirical Formulae

So far we have seen two different types of chemical formula that can be used to represent a molecule: the molecular formula and the displayed formula. Another type of formula that you need to know about is the empirical formula.

An empirical formula looks very similar to a molecular formula, but instead of showing how many atoms of each element there are in the molecule, it shows the ratio of the numbers of atoms of each element.

For example, the molecule ethane has the molecular formula C₂H₆ meaning that it is made up of two carbon atoms and six hydrogen atoms. Therefore the ratio of carbon to hydrogen atoms is 1:3, which means that ethane’s empirical formula is CH₃.

Ethane's displayed, molecular and empirical formulae.

The empirical formula should always use the simplest possible whole-number ratio. In other words, it should use the version of the ratio that has the smallest possible numbers in it.

For example, the molecule glucose has the molecular formula C₆H₁₂O₆, which means that the ratio of carbon to hydrogen to oxygen atoms is 6:12:6. One way to simplify this ratio is to divide all the numbers by 2, which gives 3:6:3.

However, this is not the simplest possible version. The simplest possible version is found if we divide all of the numbers by 6, giving the ratio 1:2:1. Therefore glucose’s empirical formula is CH₂O.

To find the simplest ratio, you need to find the highest common factor of all the numbers in the original ratio. In this case the original ratio is 6:12:6. The highest common factor of these numbers is 6, so we divide everything by 6 giving us 1:2:1. If you follow this method - finding the highest common factor and then dividing by it - you will always get the correct empirical formula.

Flashcards

Flashcards help you memorise information quickly. Copy each question onto its own flashcard and then write the answer on the other side. Testing yourself on these regularly will enable you to learn much more quickly than just reading and making notes.