Chemistry Foundations

4.3.3 - Empirical Formulae

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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 C_{2}H_{6} 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 CH_{3}.

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 C_{6}H_{12}O_{6}, 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 CH_{2}O.

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.

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What is an empirical formula?

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How do you work out an empirical formula?

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4.3.4 - Common Small Molecules

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4.3.2 - Displayed Formulae and Molecular Formulae

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