Question

An automobile engine fuel has cetane number 80. Which of the following statements is/are true?
This question has multiple correct options
(a) Fuel contains 80% of -methyl naphthalene and 20% of ${C_{16}}{H_{34}}$
(b) Fuel contains 80% of cetane and 20% of $\alpha$ -methyl naphthalene
(c) Knocking property of the given fuel compared to the knocking property of a fuel
with a cetane number of 90 is high.
(d) Cetane number determines the quality of diesel fuel in terms of ignition properties.

Answer 1

The major sources of aliphatic hydrocarbons are petroleum and natural gas, whereas aromatic hydrocarbons are mainly obtained from coal. Here we can see cetane is used which is aliphatic hydrocarbons. Cetane number is the quality or performance of a fuel. The chemical formula of cetane is ${C_{16}}{H_{34}}$.

Complete Step by step answer:
Since we have multiple answers for the question, we can go through all of the solutions one by one. The question has given us that the fuel has a certain number of 80. For that lets understand what is cetane number.
Cetane number can be defined as the quality of diesel fuel is graded in terms of cetane number. Cetane number of a diesel fuel is defined as the percentage of volume of cetane ( $n$ -hexadecane, $n - {C_{16}}{H_{34}}$ ) in a mixture of cetane and $\alpha$ -methyl naphthalene which has the same ignition properties as the fuel under consideration.

Now let us look into the options:
(a) It says the fuel contains 80% of $\alpha$ -methyl naphthalene and 20% of cetane ( ${C_{16}}{H_{34}}$ ). By definition means the cetane number is the percentage of volume of cetane in a mixture. So here the cetane percentage is 20 therefore, the cetane number of the fuel is 20. Given the fuel has a cetane number of 80. Hence option (a) is wrong.
(b) Here, the fuel contains 80% of cetane, hence by definition the cetane number is 80. In question also given that the fuel has a cetane number of 80. Hence option (b) is correct.
(c) We know that, if the cetane number is high then the knocking property will be less. Here in option it is given that the knocking property of fuel of cetane number 90 will be high. In fact, a fuel with cetane number 90 will have less knocking property as compared to a fuel with cetane number 80. Hence option (c) is wrong.
(d) In option (d) it is the direct definition of cetane number given above, hence option (d) is correct.

Option (b) and (d) is correct.

Note: Apart from cetane number there is also octane number which checks the quality of petrol or gasoline. Octane number is defined as the percentage by volume of $iso$ -octane present in the mixture of $iso$ -octane and $n$ -heptane which has the same knocking properties as the fuel under consideration.