# Why is it incorrect to write a chemical formula for petrol?

The gasoline that you put in your petrol motor is ${C}_{5} {H}_{12}$ to ${C}_{7} {H}_{16}$, a mixture of the isomeric pentanes and hexanes, and a few heptanes. The gas that you put in your diesel motor has one degree of unsaturation, i.e. ${C}_{6} {H}_{12}$ to ${C}_{10} {H}_{20}$; with the possibility of geometric isomerism, diesel is a bit of a witches' brew.
Most of the time, when we want to represent gasoline, we would use the formula ${C}_{6} {H}_{14}$, i.e. hexanes; and of course hexanes could be a mixture of isomers: $\text{dimethylbutanes}$; $\text{ethylbutane}$ $\text{methylpentanes}$; and $\text{n-hexane}$.
In the laboratory, often we use so called $\text{petroleum ethers}$, with various boiling point ranges, $40 - 60$ ""^@C, $60 - 80$ ""^@C, $100 - 120$ ""^@C. These tend to contain some unsaturated material.