What is the empirical formula of a compound that has a carbon to hydrogen ratio of 2 to 6?

Chemguide: Core Chemistry 14 - 16

This page shows how you can find formulae of compounds from experimental data. I am assuming you have already read and understood the page about moles.

Explaining what an empirical formula is

The empirical formula tells you the simplest ratio of the various atoms present in a substance. For example, in ethane, C2H6, the ratio of the number of carbon to hydrogen atoms is 1:3. The empirical formula is CH3. For hydrogen peroxide, H2O2, where the simplest ratio is 1:1, the empirical formula is HO.

The empirical formula for most molecular substances is virtually never used, unless it happens 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. In other words, you can calculate results from 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 need to know, or be able to calculate, the relative formula mass.

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.

The empirical formula of a compound is the simplest whole number ratio of the elements in the compound. In C75H25, the ratio of carbon to hydrogen is 75:25, which is equal to 3:1. Thus, the empirical formula is C3H.