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Question
If the empirical formula of two compounds is CH and their vapour densities are 13 to 39 respectively, find their molecular formula.
Solution
For acetylene , molecular mass = 2 × V.D = 2 × 13 = 26 g
The empirical mass = 12(C) + 1(H) = 13 g
n = `"Molecular formula mass"/"Empirical formula weight" = 26/13 = 2`
Molecular formula of acetylene = 2 × Empirical formula = C2H2
Similarly, for benzene molecular mass = 2 × V.D = 2 × 39 = 78
n = `78/13` = 6
So, the molecular formula = C6H6
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