What is the mass percentage of iron in Fe2O3, to three significant figures, using Fe = 55.85 and O = 16.00 g/mol?

1. Build the molar mass of Fe2O3 from its formula. 2. Iron contributes 2 x 55.85 g/mol = 111.70 g/mol. 3. Oxygen contributes 3 x 16.00 g/mol = 48.00 g/mol. 4. The molar mass is 111.70 + 48.00 = 159.70 g/mol. 5. The iron fraction is 111.70 g/mol / 159.70 g/mol. 6. Multiplying by 100 gives 69.9 percent iron to three significant figures. 7. The value 30.1 percent is the oxygen share, which is what is left over from 100 percent. 8. The value 77.7 percent comes from using only one oxygen atom, and 46.6 percent from using only one iron atom. _Source: OpenStax Chemistry (CC BY 4.0), Ch 3 "Composition of Substances and Solutions", section 3.2 Determining Empirical and Molecular Formulas_