Molar MassMolar mass of iron(II,III) oxide (Fe3O4)
The molar mass of iron(II,III) oxide, Fe3O4, is 231.53 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 231.53 grams.
231.53 g/mol
3 × 55.845 + 4 × 15.999 = 231.531 · 7 atoms in the formula · also called magnetite
About iron(II,III) oxide
Fe3O4 is magnetite, a black magnetic ore of iron. It contains iron in two oxidation states and forms when iron reacts with steam.
Other names: magnetite.
The working
The formula Fe3O4 has 3 iron atoms and 4 oxygen atoms, 7 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) | 3 | 55.845 | 167.535 | 72.36% |
| Oxygen (O) | 4 | 15.999 | 63.996 | 27.64% |
3 × 55.845 + 4 × 15.999 = 231.531 g/mol
With the rounded atomic masses used in many school books (Fe = 56, O = 16) the answer is 232 g/mol. The difference from 231.53 is too small to matter in most school sums.
Percentage composition
By mass, Fe3O4 is 72.36% iron and 27.64% oxygen. The largest share comes from iron. In a 100 g sample there are 72.4 g of iron and 27.6 g of oxygen.
Notice that oxygen has more atoms in the formula (4) than iron (3), 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 250 g of Fe3O4? Divide the mass by the molar mass: 250 ÷ 231.53 = 1.08 mol.
Moles to grams. What is the mass of 0.2 mol of Fe3O4? Multiply: 0.2 × 231.53 = 46.31 g.
Counting particles. 250 g contains 1.08 mol × 6.022 × 1023 = 6.503 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 23.15 g |
| 0.5 mol | 115.8 g |
| 1 mol | 231.5 g |
| 2 mol | 463.1 g |
| 5 mol | 1158 g |
| Mass | Moles |
|---|---|
| 1 g | 0.004319 mol |
| 10 g | 0.04319 mol |
| 100 g | 0.4319 mol |
| 1 kg | 4.319 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Fe3O4?
The molar mass of iron(II,III) oxide (Fe3O4) is 231.53 g/mol, or 231.531 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Fe3O4?
100 ÷ 231.53 = 0.4319 mol.
What is the mass of one formula unit of Fe3O4?
Divide the molar mass by the Avogadro constant: 231.53 ÷ 6.022 × 1023 = 3.845 × 10-22 g.
What is the percentage of iron in Fe3O4?
Iron makes up 72.36% of the mass of Fe3O4: 3 × 55.845 ÷ 231.531 × 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.