First, we write down the reaction
4 Fe + 3 O2 => 2 Fe2O3 (Don't forget to balance it)
We also need the molar masses:
For O2) 32.0 g/mol
For Fe) 55.8 g/mol
Procedure: We use stoichiometry
3 x 32 g of O2 (1 mole) -------------- 4 x 55.8 g of Fe (1 mole)
X -------------- 25.0 g of Fe
X = 10.8 g of O2 are needed
Answer: 10.8 g of O2
