The molar mass of iron(III) oxide (Fe2O3) is 159.687 g/mol.
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or
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| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Iron (Fe) | 2 | 55.845 | 111.690 | 69.94% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 30.06% |
| Total | 5 | 159.687 | 100.00% |
111.690 + 47.997 = 159.687 g/mol
Grams and moles
25 g of iron(III) oxide is 0.1566 mol, because 25 ÷ 159.687 = 0.1566.
This element has no single atomic weight. Their isotopic mix varies slightly depending on where on Earth the sample came from, so the figure used here is the conventional value IUPAC publishes for when one number is needed: oxygen is 15.999, and really lies between 15.99903 and 15.99977.
Iron(III) oxide is rust, hematite, and the red pigment in everything from Mars to barn paint. It weighs 159.687 g/mol, with iron contributing 111.690 and oxygen 47.997, making it 69.94% iron by mass.
That percentage is the number an ore body is valued on. Hematite ore assaying at 60% Fe2O3 contains about 42% iron, and smelting economics run on the difference.
The Roman numeral is doing real work in the name. Iron(II) oxide is FeO at 71.844 g/mol and is black; iron(III) oxide is red. There is also magnetite, Fe3O4 at 231.531 g/mol, which contains iron in both oxidation states at once and is the ore that made a compass needle possible. Three different oxides of the same two elements, three different molar masses, three different substances.
Writing iron oxide without a numeral does not identify which one you mean, and the masses differ by more than a factor of three.