Crude oil is a finite resource. It can be separated into its useful fractions by fractional distillation. Some fractions undergo cracking to help meet demand for the smaller molecules.

Crude oil is mostly made up of a group of hydrocarbons called the alkanes.

A homologous series is a series of molecules which:

- have the same general formula
- differ by CH
_{2}in the molecular formula of neighbouring compounds - show a gradual variation in physical properties
- have similar chemical properties

The table shows four alkanes, their molecular formula, displayed formula and ball-and-stick model diagram.

The formula of any alkane can be worked out using the general formula C_{n}H_{2n+2} where n = the number of carbon atoms.

For example, for a hydrocarbon with ten carbon atoms:

- n = 10 and 2n+2 = 22
- the formula is C
_{10}H_{22}

- Question
Work out the formula for a hydrocarbon molecule with 6 carbon atoms.

C

_{6}H_{14}