The standard chemical formula for petrol is CnH2n+2. This formula refers to alkanes, and the chemical formula for petrol can be altered when it consists of different gases.
The actual formula of petrol will differ slightly as petroleum is refined. For example, the above formula will change when pentane is changed into octane, and is further changed into diesel fuel, kerosene and jet fuel. The alkanes are also known as paraffins, and contain only hydrogen and oxygen as saturated hydrocarbons. As petrol is refined, the formula will change. For example, the formula for cycloalkanes is CnH2n, and the formula for aromatic hydrocarbons is CnHn.