Petrol (or gasoline) is not a single molecule, but a complex mixture of hydrocarbons with varying chain lengths and structures.

Therefore, there is no single molecular formula for petrol.

The main components of petrol are:

* Alkanes: These are straight-chain hydrocarbons with the general formula C_nH_2n+2, where 'n' represents the number of carbon atoms. Examples include octane (C₈H₁₈), heptane (C₇H₁₆), and hexane (C₆H₁₄).

* Cycloalkanes: These are alkanes with a ring structure. Examples include cyclohexane (C₆H₁₂) and cyclopentane (C₅H₁₀).

* Aromatic hydrocarbons: These hydrocarbons contain a benzene ring (C₆H₆) structure. Examples include toluene (C₇H₈) and xylene (C₈H₁₀).

The composition of petrol varies depending on the source of the crude oil and the refining process.

It's important to note that the term "octane number" is often used to describe the quality of petrol. It's a measure of the fuel's resistance to knocking or premature detonation in an internal combustion engine. However, octane number is not a molecular formula.