Calculate the relative molecular weights: a) iron oxide (III); b) phosphorus (V) oxide c) manganese (VII) oxide.

The relative molecular weight of a substance shows the ratio of the mass of its molecule to 1/12 of the mass of a carbon atom. Relative atomic mass is dimensionless. Let’s calculate the relative molecular weights of the following substances: a) iron (III) oxide (Fe2O3); b) phosphorus (V) oxide (P2O5) c) manganese (VII) oxide (Mn2O7)
Mr (Fe2O3) = 56 * 2 + 16 * 3 = 160
Mr (P2O5) = 31 * 2 + 16 * 5 = 142
Mr (Mn2O7) = 55 * 2 + 16 * 7 = 222
Answer: Mr (Fe2O3) = 160, Mr (P2O5) = 142, Mr (Mn2O7) = 222