While a mixture of aluminum powder and iron (lll) oxide is ignited, it produces molten iron as well as aluminum oxide. In an experiment, 5.40g of aluminum was reacted with 18.50 g of iron (lll) oxide. At the end of the reaction, the mixture contained 11.17g iron, 10.20 g of aluminum oxide and an undetermined amount of unreacted iron (lll) oxide. No aluminum was left. Explain what is the mass of the iron (lll) oxide?