WAEC 2023 Chemistry · Question 7

The relative molar mass of a gaseous hydrocarbon is 30. Determine its vapour density.

Options

A.45
B.30
C.15
D.60

Answer: C. 15

Explanation written with AI and checked against the answer key

Analyze the question: You're given the relative molar mass of a gaseous hydrocarbon (30) and asked for its vapour density. Concept: Vapour density (V.D.) compares the mass of a gas to the mass of an equal volume of hydrogen under the same conditions: V.D. = Molar mass of gas ÷ Molar mass of H₂ Since H₂ has a molar mass of 2 (2 × 1), this gives the key relationship: Relative molar mass = 2 × Vapour density so V.D. = Relative molar mass ÷ 2 Solution: V.D. = 30 ÷ 2 = 15 Check: The hydrocarbon with a molar mass of 30 is ethane (C₂H₆): (2 × 12) + (6 × 1) = 30. Its vapour density is 30/2 = 15. ✔ Answer: C. 15 Exam tip: Remember "Molar mass is double the vapour density" (M = 2 × V.D.). Students often pick 30 by confusing the two, or choose a larger number by multiplying instead of dividing. Vapour density has no units because it is a ratio. If the question compares to air instead, use the molar mass of air (about 29).

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