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:

=%ofironbymassAtomicmassoroxygen
=69.955.85=1.25

Relative moles of oxygen in iron oxide:

=%ofoxygenbymassAtomicmassoroxygen
=30.116.00=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 Fe2O3