Ncert Exercises & Solutions

The empirical formula of an oxide of iron, which has 69.9% iron and 30.1% dioxygen by mass is:

1.	FeO	2.	Fe₂O₃
3.	Fe₃O₄	4.	Fe₃O₂

Hint: Use the empirical formula method

Step 1:

% of iron by mass = 69.9 %
% of oxygen by mass = 30.1 %

Relative
moles of iron in iron oxide:

$= \frac{% of iron by mass}{Atomic mass or oxygen}$
$= \frac{69.9}{55.85} = 1.25$

Relative moles of oxygen in iron oxide:

$= \frac{% of oxygen by mass}{Atomic mass or oxygen}$
$= \frac{30.1}{16.00} = 1.88$

Step 2:

The simplest molar ratio of iron to oxygen:

= 1.25: 1.88
= 1: 1.5
= 2: 3
$∴$ The empirical formula of the iron oxide is ${Fe}_{2} O_{3}$