Question

What is the source of an alkane if its molar mass is 240 g mol$^{-1}$ and the percentage by mass of hydrogen is 15%?

• A. Gasoline
• B. Diesel
• C. Petrol
• D. Coatings on green leaves

Hint:

To identify the source of an alkane, calculate the empirical formula and match it with typical alkane sources such as diesel.

Answer 1

The Correct Option is B

Step 1: Determining the empirical formula.
Given that the percentage of hydrogen is 15%, the percentage of carbon is 85%. The molecular mass of hydrogen is 1 g/mol and carbon is 12 g/mol. Therefore, the number of moles of carbon and hydrogen in the molecule are: \[ \text{Moles of C} = \frac{85}{12} \approx 7.08, \quad \text{Moles of H} = \frac{15}{1} = 15 \] This gives us the empirical formula \( \text{C}_7\text{H}_{15} \), which corresponds to an alkane.
Step 2: Analyzing the options.
(A) Gasoline: Incorrect — gasoline has a different molecular formula and molar mass.
(B) Diesel: Correct — Diesel is a mixture of heavier alkanes, fitting the molecular mass.
(C) Petrol: Incorrect — petrol has a different molecular formula and molar mass.
(D) Coatings on green leaves: Incorrect, this is unrelated to alkanes.
Step 3: Conclusion.
The correct answer is (B) Diesel.