Where the mass fraction of iron is greater: in iron oxide (2) or in iron oxide (3).

Given:
FeO
Fe2O3

To find:
ω (Fe in FeO) -?
ω (Fe in Fe2O3) -?

Decision:
1) Calculate the molar masses of FeO and Fe2O3:
Mr (FeO) = Ar (Fe) + Ar (O) = 56 + 16 = 72 g / mol;
Mr (Fe2O3) = Ar (Fe) * 2 + Ar (O) * 3 = 56 * 2 + 16 * 3 = 160 g / mol;
2) Calculate the mass fraction of Fe in FeO:
ω (Fe in FeO) = N (Fe) * Ar (Fe) * 100% / Mr (FeO) = 1 * 56 * 100% / 72 = 77.8%;
3) Calculate the mass fraction of Fe in Fe2O3:
ω (Fe in Fe2O3) = N (Fe) * Ar (Fe) * 100% / Mr (Fe2O3) = 2 * 56 * 100% / 160 = 70%;
4) ω (Fe in FeO)> ω (Fe in Fe2O3).

Answer: ω (Fe in FeO)> ω (Fe in Fe2O3).