Question

Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R Assertion A: The octane number which is a measure of knock characteristic of fuel is derived from a mixture of Iso-octane and Normal Heptane Reason R: The normal Heptane has the octane number as 100. In the light of the above statements, choose the most appropriate answer from the options given below

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

For octane number scale: \[ \text{Iso-octane} \rightarrow 100 \] \[ \text{Normal heptane} \rightarrow 0 \] Higher octane number means better anti-knocking property in SI engines.

Answer 1

The Correct Option is C

Concept: Octane number is a very important property of petrol fuels. It indicates the anti-knocking quality of fuel used in Spark Ignition engines. Knocking is an abnormal combustion phenomenon that causes:
• vibration,
• loss of power,
• overheating,
• engine damage. To define octane number, two standard reference fuels are used:
• Iso-octane
• Normal heptane Iso-octane possesses excellent anti-knock properties whereas normal heptane has very poor anti-knock quality.

Step 1: Analyzing Assertion A. Assertion A states: \[ \text{``The octane number is derived from a mixture of Iso-octane and Normal Heptane''} \] This statement is scientifically correct. Octane number is determined by comparing the knocking tendency of a fuel with mixtures of: \[ \text{Iso-octane} + \text{Normal heptane} \] Reference values are assigned as: \[ \text{Iso-octane} = 100 \] \[ \text{Normal heptane} = 0 \] For example:
• A fuel behaving like a mixture containing \(90%\) iso-octane and \(10%\) heptane has octane number 90. Hence Assertion A is: \[ \boxed{\text{Correct}} \]

Step 2: Analyzing Reason R. Reason R states: \[ \text{``The normal Heptane has the octane number as 100''} \] This statement is incorrect. Actually: \[ \text{Iso-octane} = 100 \] whereas: \[ \text{Normal heptane} = 0 \] Normal heptane is highly knock-prone and therefore assigned the lowest octane value. Hence Reason R is: \[ \boxed{\text{Incorrect}} \]

Step 3: Selecting the correct option. We found that:
• Assertion A is true
• Reason R is false Therefore the correct answer is: \[ \boxed{\text{Option (C)}} \] \[ \boxed{\text{A is correct but R is not correct}} \] Verification Table: \[ \begin{array}{|c|c|} \hline \text{Statement} & \text{Result} \hline \text{Assertion A} & \text{Correct} \hline \text{Reason R} & \text{Incorrect} \hline \end{array} \] Important Reference Values: \[ \text{Iso-octane} = 100 \] \[ \text{Normal heptane} = 0 \]