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Question
A compound made up of two elements X and Y has an empirical formula X2Y. if the atomic weight of X is 10 and that Y is 5 and the compound has a vapour density (V.D.) 25, find its molecular formula.
Solution
Empirical formula = X2Y
Atomic weight (X) = 10
Atomic weight (Y) = 5
Empirical formula weight = (2 × 10) + 5 = 25
n = `"Molecular weight"/"Empirical formula weight"`
= `[2 xx "V.D."]/"Empirical formula weight"`
= `[ 2 xx 25]/25`
= 2
∴ Molecular formula = X2Y × 2
= X4Y2
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