Molar MassMolar mass of aluminium oxide (Al2O3)
The molar mass of aluminium oxide, Al2O3, is 101.96 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 101.96 grams.
101.96 g/mol
2 × 26.982 + 3 × 15.999 = 101.961 · 5 atoms in the formula · also called alumina
About aluminium oxide
Aluminium oxide is alumina, obtained from bauxite and electrolysed to give aluminium metal. Ruby and sapphire are crystals of it coloured by traces of other metals.
Other names: alumina.
The working
The formula Al2O3 has 2 aluminium 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 |
|---|---|---|---|---|
| Aluminium (Al) | 2 | 26.982 | 53.964 | 52.93% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 47.07% |
2 × 26.982 + 3 × 15.999 = 101.961 g/mol
With the rounded atomic masses used in many school books (Al = 27, O = 16) the answer is 102 g/mol. The difference from 101.96 is too small to matter in most school sums.
Percentage composition
By mass, Al2O3 is 52.93% aluminium and 47.07% oxygen. The largest share comes from aluminium. In a 100 g sample there are 52.9 g of aluminium and 47.1 g of oxygen.
Notice that oxygen has more atoms in the formula (3) than aluminium (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 20 g of Al2O3? Divide the mass by the molar mass: 20 ÷ 101.96 = 0.1962 mol.
Moles to grams. What is the mass of 0.5 mol of Al2O3? Multiply: 0.5 × 101.96 = 50.98 g.
Counting particles. 20 g contains 0.1962 mol × 6.022 × 1023 = 1.181 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 10.2 g |
| 0.5 mol | 50.98 g |
| 1 mol | 102 g |
| 2 mol | 203.9 g |
| 5 mol | 509.8 g |
| Mass | Moles |
|---|---|
| 1 g | 0.009808 mol |
| 10 g | 0.09808 mol |
| 100 g | 0.9808 mol |
| 1 kg | 9.808 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of Al2O3?
The molar mass of aluminium oxide (Al2O3) is 101.96 g/mol, or 101.961 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of Al2O3?
100 ÷ 101.96 = 0.9808 mol.
What is the mass of one formula unit of Al2O3?
Divide the molar mass by the Avogadro constant: 101.96 ÷ 6.022 × 1023 = 1.693 × 10-22 g.
What is the percentage of aluminium in Al2O3?
Aluminium makes up 52.93% of the mass of Al2O3: 2 × 26.982 ÷ 101.961 × 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.