Formulna mass of Fe2O3 = (2 atomic mass of Fe) + (3 atomic mass of O)

Atomic mass of Fe = 55.845 g/mol
Atomic mass of O = 15.999 g/mol

Formulna mass of Fe2O3 = (2 55.845) + (3 15.999)
Formulna mass of Fe2O3 = 111.69 + 47.997
Formulna mass of Fe2O3 = 159.687 g/mol

Therefore, the formulna mass of Fe2O3 is 159.687 g/mol.