A compound of X and Y has the empirical formula XY₂. Its vapour density is equal to its empirical formula weight. Determine its molecular formula.
Topic: Mole concept, gas volumes and stoichiometry
Answer
Empirical formula = XY₂, so empirical formula weight = x + 2y (call it E).
Given: vapour density = E.
Molecular mass = 2 × vapour density = 2 × E
n = molecular mass ÷ empirical formula weight = 2E ÷ E = 2
Molecular formula = (XY₂)ₙ = (XY₂)₂ = X₂Y₄
Molecular formula = X₂Y₄
Two facts carry the whole question. Molecular mass = 2 × vapour density, and the molecular formula is the empirical formula multiplied by n, where n = molecular mass ÷ empirical formula mass.
Because the vapour density happens to EQUAL the empirical formula weight, the letters cancel and n = 2 whatever X and Y actually are. You never need their atomic masses — that is the point of the question. A student who stalls looking for values of X and Y has missed it. Forgetting the factor of 2 gives n = 1 and the answer XY₂, which is the empirical formula, not the molecular one.