The vapour density of an organic compound with the molecular formula $C_2H_4O_2$, is [H=1.0, C=12.0, O= 16.0]

Analyze the question: You're asked for the vapour density of C₂H₄O₂. Vapour density (V.D.) compares the density of a gas or vapour to that of hydrogen.

Concept:

Vapour density = Molar mass ÷ 2

(This is because Molar mass = 2 × Vapour density, since a hydrogen molecule H₂ has a mass of 2.)

Solution:

1. Calculate the molar mass of C₂H₄O₂: • C: 2 × 12 = 24 • H: 4 × 1 = 4 • O: 2 × 16 = 32 • Total = 24 + 4 + 32 = 60 g/mol

2. Calculate the vapour density: • V.D. = 60 ÷ 2 = 30

Answer: Option D (30).

Tip: Whenever you see "vapour density" in an exam, just find the molar mass and halve it. Also, the reverse works: if the question gives V.D. and asks for molar mass, double it. Watch out for option B (120), which is what you'd get if you multiplied by 2 instead of dividing.