Molar MassMolar mass of iron(III) oxide (Fe2O3)
The molar mass of iron(III) oxide, Fe2O3, is 159.69 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 159.69 grams.
159.69 g/mol
2 × 55.845 + 3 × 15.999 = 159.687 · 5 atoms in the formula · also called ferric oxide, haematite
About iron(III) oxide
Iron(III) oxide is the main part of the ore haematite and of rust. Iron is extracted from it in the blast furnace. Finding the percentage of iron in it is a very common exam question.
Other names: ferric oxide, haematite.
The working
The formula Fe2O3 has 2 iron atoms and 3 oxygen atoms, 5 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Iron (Fe) | 2 | 55.845 | 111.690 | 69.94% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 30.06% |
2 × 55.845 + 3 × 15.999 = 159.687 g/mol
With the rounded atomic masses used in many school books (Fe = 56, O = 16) the answer is 160 g/mol. The difference from 159.69 is too small to matter in most school sums.
Percentage composition
By mass, Fe2O3 is 69.94% iron and 30.06% oxygen. The largest share comes from iron. In a 100 g sample there are 69.9 g of iron and 30.1 g of oxygen.
Notice that oxygen has more atoms in the formula (3) than iron (2), yet gives a smaller share of the mass. Percentage by mass depends on how heavy each atom is, not only on how many there are.
Worked examples
Grams to moles. How many moles are in 100 g of Fe2O3? Divide the mass by the molar mass: 100 ÷ 159.69 = 0.6262 mol.
Moles to grams. What is the mass of 2 mol of Fe2O3? Multiply: 2 × 159.69 = 319.4 g.
Counting particles. 100 g contains 0.6262 mol × 6.022 × 1023 = 3.771 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 15.97 g |
| 0.5 mol | 79.84 g |
| 1 mol | 159.7 g |
| 2 mol | 319.4 g |
| 5 mol | 798.4 g |
| Mass | Moles |
|---|---|
| 1 g | 0.006262 mol |
| 10 g | 0.06262 mol |
| 100 g | 0.6262 mol |
| 1 kg | 6.262 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Fe2O3?
The molar mass of iron(III) oxide (Fe2O3) is 159.69 g/mol, or 159.687 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Fe2O3?
100 ÷ 159.69 = 0.6262 mol.
What is the mass of one formula unit of Fe2O3?
Divide the molar mass by the Avogadro constant: 159.69 ÷ 6.022 × 1023 = 2.652 × 10-22 g.
What is the percentage of iron in Fe2O3?
Iron makes up 69.94% of the mass of Fe2O3: 2 × 55.845 ÷ 159.687 × 100.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.