When a mixture of aluminum powder and iron(iii) oxide is ignited, it produces molten iron and aluminum oxide. in an experiment, 5.40 g aluminum was mixed with 18.50 g iron(iii) oxide. at the end of the reaction, the mixture contained 11.17 g iron, 10.20 g aluminum oxide, and an undetermined amount of unreacted iron(iii) oxide. no aluminum was left. what is the mass of the unreacted iron(iii) oxide?