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Question
Find the molecular formula of a hydrocarbon having vapour density 15, which contains 20% of Hydrogen.
Solution
% of hydrogen = 20%
% of carbon = 100 − 20 = 80%
|
Element |
% Weight |
Atomic Weight |
Relative No. of Moles |
Simplest Ratio |
|
C |
80 |
12 |
`80/12` = 6.667 |
`6.667/6.667` = 1 |
|
H |
20 |
1 |
`20/1` = 20 |
`20/6.667` = 2.99 ≈ 3 |
Empirical formula = CH3
Empirical formula weight = 1 x 12 + 1 x 3 = 12 + 3 = 15
Vapour Density = 15
Relative molecular mass = 15 x 2 = 30
n = `"Relative molecular mass"/"Empirical Weight" = 30/15 = 2`
Molecular formula = n x empirical formula
= 2 x CH3
= C2H6
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