Question

A gaseous compound of nitrogen and hydrogen contains 12.5%  hydrogen by mass. Find the molecular formula of the compound if its relative molecular mass is 37.

[N = 14, H = 1].

Answer 1

Calculation of empirical formula:

Percentage of hydrogen = 12.5%

Percentage of nitrogen = 100 − 12.5 = 87.5%

Element	Atomic weight	Percentage	Relative Number of moles	Simplest whole no ratio
N	14	87.5	`87.5/14` = 6.25	`6.25/6.25` = 1
H	1	12.5	`12.5/1` =12.5	`12.5/6.25` = 2

Empirical formula = NH₂

Given molecular mass = 37

Empirical formula mass = 14 + 2 = 16

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

= `37/16`

∴ n = 2.31

Hence, Molecular Formula = (NH₂)_2.31

= N_2.31H_4.62