How many moles of iron are needed to react with 0.125 moles of oxygen
Given; 4Fe + 3O2 = 2Fe2O3

0.167 mol Fe

Explanation:

4 Fe  +  3 O₂  ⇒  2 Fe₂O₃

Convert moles of oxygen to moles of iron using the mole ratio.  You can find the mole ratio using the chemical equation.  For every 4 moles of iron, you will need 3 moles of oxygen.

(0.125 mol O₂)  (4 mol Fe/3 mol O₂) = 0.167 mol Fe

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)