The molar mass of iron(III) oxide, Fe2O3, is 159.687 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 159.687 g, and a gram of it is 3.771e+21 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Iron(III) oxide is 2 × 55.845 (Fe) + 3 × 15.999 (O), which is 159.687 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Iron makes up the largest share of the mass: 2 of the 5 atoms in a formula unit are iron, and they account for 111.69 g of the 159.687 g, or 69.94% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of iron(III) oxide is 159.687 mg and one micromole is 159.687 µg, so a balance reading to a milligram places you within 6.262 µmol. A single formula unit weighs 2.6517e-22 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Rust, and the red pigment in everything from bricks to thermite. Its molar mass is how ore grade is converted into iron content. At 159.687 g/mol it is the heaviest of the 8 oxides covered here, and that ranking matters more than it looks: aluminium oxide has a molar mass of 101.9601 g/mol, so a gram of it contains 56.6% more formula units than a gram of iron(III) oxide. Weigh by mass and you are not weighing equal amounts of substance.
Two practical things move the result. Purity below 100% means the weighed mass overstates how much compound you have — at 98% assay, 3.1937 g of every 159.687 g is not the compound — and any water picked up from the air does the same. Both are systematic, so they shift every solution made from that jar in the same direction.
The formula
m- The mass of iron(III) oxide in grams
159.687- The molar mass of iron(III) oxide in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of iron(III) oxide in grams.
- It is divided by the molar mass, 159.687 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of iron(III) oxide as a molecule count
0.5 g divided by 159.687 g/mol is 0.00313113 mol, and multiplying by the Avogadro constant gives 1.886e+21 formula units of iron(III) oxide. The same mass is 3.1311 mmol, which is the figure a reaction is actually planned in.
5 g of iron(III) oxide as a molecule count
5 g divided by 159.687 g/mol is 0.0313113 mol, and multiplying by the Avogadro constant gives 1.886e+22 formula units of iron(III) oxide. The same mass is 31.311 mmol, which is the figure a reaction is actually planned in.
50 g of iron(III) oxide as a molecule count
50 g divided by 159.687 g/mol is 0.313113 mol, and multiplying by the Avogadro constant gives 1.886e+23 formula units of iron(III) oxide. The same mass is 313.11 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of iron(III) oxide (Fe2O3)?
It is 159.687 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 55.845 (Fe) + 3 × 15.999 (O). One mole of iron(III) oxide therefore weighs 159.687 g, and a gram of it is 6.2623 mmol.
What percentage of iron(III) oxide is iron?
69.94% by mass. Each formula unit contains 2 iron atoms contributing 111.69 g of the 159.687 g total, so 100 g of iron(III) oxide contains 69.94 g of iron and a kilogram contains 699.43 g of it.
How many molecules are in one gram of iron(III) oxide?
About 3.771e+21. One gram is 0.00626225 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 159.687.
How much does a single molecule of iron(III) oxide weigh?
2.6517e-22 g, which is 159.687 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for iron(III) oxide it is 159.687. Molar mass carries grams per mole: 159.687 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Iron is quoted as 55.845 rather than a whole number for that reason, and the same applies to the other elements here, which is why iron(III) oxide comes out at 159.687 g/mol rather than 160.
Molar mass, composition and mole equivalents of iron(III) oxide
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Iron (Fe) | 2 | 55.845 | 111.69 | 69.94% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 30.06% |
| Total — one mole of iron(III) oxide | 159.687 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 159.687 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000159687 | 0.159687 | 6.022e+17 |
| 10 µmol | 0.00159687 | 1.59687 | 6.022e+18 |
| 100 µmol | 0.0159687 | 15.9687 | 6.022e+19 |
| 1 mmol | 0.159687 | 159.687 | 6.022e+20 |
| 10 mmol | 1.59687 | 1596.87 | 6.022e+21 |
| 50 mmol | 7.98435 | 7984.35 | 3.011e+22 |
| 100 mmol | 15.9687 | 15,968.7 | 6.022e+22 |
| 250 mmol | 39.9218 | 39,921.8 | 1.506e+23 |
| 500 mmol | 79.8435 | 79,843.5 | 3.011e+23 |
| 750 mmol | 119.765 | 119,765 | 4.517e+23 |
| 1 mol | 159.687 | 159,687 | 6.022e+23 |
| 2 mol | 319.374 | 319,374 | 1.204e+24 |
| 5 mol | 798.435 | 798,435 | 3.011e+24 |
| 10 mol | 1596.87 | 1596870.000000 | 6.022e+24 |
Mass is the amount multiplied by 159.687 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Silicon dioxide | SiO2 | 60.083 | 16.644 |
| Copper(II) oxide | CuO | 79.545 | 12.572 |
| Titanium dioxide | TiO2 | 79.865 | 12.521 |
| Zinc oxide | ZnO | 81.379 | 12.288 |
| Manganese dioxide | MnO2 | 86.93604 | 11.503 |
| Aluminium oxide | Al2O3 | 101.9601 | 9.8078 |
| Iron(III) oxide (this page) | Fe2O3 | 159.687 | 6.2623 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.