A sample of petrol contains 30% n-heptane. Its octane number is

Options

A. 30
B. 70
C. 15
D. 45

Explanation

To determine the octane number of a petrol sample that contains 30% n-heptane, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Components of Petrol**:
- Petrol is primarily a mixture of n-heptane and iso-octane.
- n-Heptane has an octane number of 0, while iso-octane has an octane number of 100.

2. **Identify the Percentage of n-Heptane**:
- The problem states that the sample contains 30% n-heptane.

3. **Calculate the Percentage of Iso-Octane**:
- Since the total composition must add up to 100%, we can find the percentage of iso-octane:
\[ \text{Percentage of iso-octane} = 100\% - \text{Percentage of n-heptane} = 100\% - 30\% = 70\% \]

4. **Determine the Octane Number**:
- The octane number of the petrol sample is directly related to the percentage of iso-octane present in the mixture. Therefore, the octane number can be taken as the percentage of iso-octane:
\[ \text{Octane Number} = \text{Percentage of iso-octane} = 70 \]

5. **Conclusion**:
- The octane number of the petrol sample is 70.

### Final Answer:
The octane number of the petrol sample is **70**.

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