Question

A compound A consists of 4.8% carbon and 95.2% bromine by mass. Its vapour density is 252. Find its:

1. empirical formula
2. molecular formula

Answer 1

Element	% composition	Atomic weight	Relative no. of atoms	Simplest ratio
Carbon	4.8	12	`4.8/12 = 0.4`	`0.4/0.4 = 1`
Bromine	95.2	80	`95.2/80 = 1.19`	`1.19/0.4 = 2.9 = 3`

Simplest ratio of whole numbers = C : Br = 1 : 3

Hence, empirical formula is CBr₃

i. Empirical formula weight = 12 + 3(80) = 252 g

Molecular weight = 2 × V.D. = 2 × 252 = 504

n = `"Molecular weight"/"Empirical formula weight"`

= `504/252`

= 2

ii. Molecular formula = n[E.F.] = 2[CBr₃] = C₂Br₆

∴ Molecular formula = C₂Br₆