Chemguide: Support for CIE A level Chemistry
Learning outcome 1.4(a)
This statement deals with the meaning of the words "empirical formula" and "molecular formula".
Before you go on, you should find and read the statement in your copy of the syllabus.
This can obviously only apply to substances which consist of molecules - substances with a fixed number of atoms joined together by covalent bonds. (Or in the case of the noble gases, a molecule can also be a single atom.) The term "molecular formula" can't be applied to ionic substances like sodium chloride.
The molecular formula of the hydrocarbon ethane is C2H6, for example, and the molecular formula of hydrogen peroxide is H2O2.
The molecular formula gives a precise count of the numbers of each sort of atom which make up the molecule.
By contrast, the empirical formula tells you the simplest ratio of the various atoms present in a substance. For example, in ethane (above) the ratio of the number of carbon to hydrogen atoms is 1:3. The empirical formula is CH3. For hydrogen peroxide, where the simplest ratio is 1:1, the empirical formula is HO.
The empirical formulae for most molecular substances are virtually never used, unless they happen to be the same as the molecular formula - as in H2O, for example. So what is the point of them?
The word "empirical" means "derived from observation or experiment" - so an empirical formula is one which you can find by doing experiments, as you will see in Learning Outcome 1(f). In other words, you can do an experiment which will tell you that the empirical formula of a particular hydrocarbon (not ethane) is CH2, for example.
That in itself isn't very helpful. The hydrocarbon could be C2H4, C3H6, C4H8, and so on and so on - anything with a carbon to hydrogen ratio of 1:2. To find out the correct molecular formula from the empirical formula, you would have to do further experiments. You will find out how to do that in Outcome 1(f) as well.
The empirical formula is just a stage on the way to finding out the molecular formula of something.
The empirical formula and ionic compounds
For ionic compounds, like sodium chloride, the formula quoted is almost always the empirical formula. In an ionic compound, there are no fixed numbers of ions - it depends on how big the crystal is. So the formula of sodium chloride is simply given as NaCl, showing the 1:1 ratio. The formula of sodium oxide is Na2O, showing a 2:1 ratio.
Note: You may come across a few ionic compounds such as sodium peroxide, Na2O2, or mercury(I) chloride, Hg2Cl2, where the formula normally used isn't the empirical formula. There are good reasons for this which would be unnecessarily confusing to discuss now. Almost all ionic compounds use the empirical formula.
In a real crystal of sodium chloride or sodium oxide, there will be some huge variable number of positive and negative ions. The formula we write just tells us what the ratio is.
© Jim Clark 2010 (last modified March 2014)