The petrol, octane number of which is 80, has: A.20% n-heptane + 80% iso-octane B.20% n-heptane + 80% n-octane C.80% n-heptane + 20% iso-octane D.80% n-heptane + 20% n-octane
Hint: Whenever the octane number of any sample of petrol is mentioned, it basically stands for the amount of iso-octane present in the fuel. The remaining composition consists of n-heptane.
Complete step by step answer: -Petrol is made up of a number of hydrocarbons. It is widely used as a fuel for petrol engine cars. When the fuel is pumped into a cylinder of the engine, a spark plug is used to ignite the fuel. This ignition causes the fuel to burn and push the piston upwards, thus generating the necessary force to run the car. -While this process may sound easy at first, there are quite a few problems we can face while completing this process. One of the most important and widely discussed problems is known as “Knocking”. Knocking is related to the type of combustion caused by the kind of fuel used. -This means that when fuel of lower quality is used, the ignition of the fuel is not smooth and is non uniform, which causes the rattling of the piston in the cylinder. This creates a specific kind of noise known as knocking. In the long run, knocking manages to reduce the life of the engine significantly and is hence an undesired property. -Hence, to monitor the quality of the fuel used, a parameter known as octane number was introduced. It is basically a measure of knock resistance of the fuel used. The octane number is calculated by comparing the characteristics of a gasoline to isooctane (2,2,4-trimethylpentane) and heptane. Iso-octane has low knock characteristics whereas n-heptane has a high knocking tendency. -Whenever the octane number of any sample of petrol is mentioned, it basically stands for the amount of iso-octane present in the fuel. The remaining composition consists of n-heptane. -Hence, the petrol, octane number of which is 80, has 20% n-heptane + 80% iso-octane.
Hence, Option A is the correct option.
Note: Octane ratings decrease with increasing carbon chain length. Octane ratings increase with carbon chain branching. Octane ratings increase in aromatics with the same number of carbons.