Question

A metal M forms a volatile chloride containing 65.5% chlorine. If the density of the chloride relative to hydrogen is 162.5, find the molecular formula of the chloride (M = 56).

Answer 1

% age of M in the chloride = 100 − 65.5 = 34.5

Element	Mass % age	Atomic mass	Rel. number of atoms	Simplest ratio
M	34.5	56	`34.5/56` = 0.6	`0.61/0.61` = 1
Cl	65.5	35.5	`65.5/35.5` = 1.84	`18.4/0.6` = 3

The empirical formula of chloride = MCl₃

The empirical formula of mass = 56 + 35.5 × 3

= 162.5 g

Vapour density = 162.5

Molecular density = 2 × vapour density 

= 2 × 162.5

= 325.0 g

Molecular Formula = (Empirical formula) × n

and n = `"Molecular mass"/"Empirical formula mass"`

= `325.0/162.5`

= 2

∴ Molecular Formula = (MCl₃)₂ or M₂Cl₆