Question

A hydrocarbon contains 82.8% of carbon. Find it’s molecular formula if it’s vapour density is 29 [H = 1, C = 12]

August 2, 2024

Answer

Element	% composition	At. wt.	Relative no. of atoms	Simplest ratio
Carbon	82.8	12	= 6.9	= 1 × 2 = 2
Hydrogen	100 – 82.8 = 17.2	1	= 17.2	= 2.5 × 2 = 5

C : H = 2 : 5

Simplest ratio of whole numbers = 2 : 5

Hence, the empirical formula is C₂H₅

Empirical formula weight = 2(12) + 5(1)

= 24 + 5 = 29

Vapour density (V.D.) = 29

Molecular weight = 2 × V.D.

= 2 × 29 = 58

n = $Molecular\ weight\over Emperical\ Formula\ Weight$

= $58\over 29$ = 2

Molecular formula = (C₂H₅)₂ = C₄H₁₀

Hence, Molecular formula = C₄H₁₀

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