Question

An organic compound with vapour density = 94 contains C = 12.67%, H = 2.13% and Br = 85.11%. Find the molecule formula.

[Atomic mass: C = 12, H = 1, Br = 80]

Answer 1

Element	Symbol	Atomic No.	Percentage	Relative no. of atoms	Simplest ratio
Hydrogen	H	1	2.13	`2.13/1` = 2.13	`2.13/1.055` = 2
Carbon	C	12	12.67	`12.67/12` = 1.055	`1.055/1.055` = 1
Bromine	Br	80	85.11	`85.11/80` = 1.0638	`1.0638/1.055` = 1

Empirical formula = CH₂Br

Given: Vapour density = 94

Molecular mass = 2 × Vapour density

= 2 × 94

= 188

Molecular formula mass = Empirical formula mass × n

188 = 94 × n

n = `188/94`

= 2

Molecular formula = (Empirical formula) × n

= (CH₂Br) × 2

= C₂H₄Br₂