Petrol-range hydrocarbons are typically in which carbon number range?

• A. C3 to C7
• B. Approximately C5 to C10
• C. C11 to C20
• D. C1 to C4

Answer: (B)

The difficulty here is understanding how the size of a hydrocarbon molecule affects its volatility and suitability as a fuel. Petrol (gasoline) needs hydrocarbons that are liquid at room temperature but still volatile enough to vaporize readily in an engine. As the carbon chain gets longer, the boiling point rises and the compound becomes less volatile; as the chain gets shorter, it becomes more volatile and often gases at room temperature.

Mid-length hydrocarbons—roughly five to ten carbon atoms—strike the right balance. They are liquids at ambient conditions and boil in a range that allows rapid vaporization in the engine without being so light that they would all escape as gas. Shorter hydrocarbons (C1–C4) are gases at room temperature and would not form the liquid fuel we need, while longer hydrocarbons (C11–C20 and beyond) are too heavy and less volatile for petrol, tending toward diesel or heavier fuels.

So petrol-range hydrocarbons are approximately C5 to C10.