The molar mass of aluminum oxide (Al2O3) is 101.960 g/mol.
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| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Aluminum (Al) | 2 | 26.9815384 | 53.963 | 52.93% |
| Oxygen (O) | 3 | 15.999 | 47.997 | 47.07% |
| Total | 5 | 101.960 | 100.00% |
53.963 + 47.997 = 101.960 g/mol
Grams and moles
25 g of aluminum oxide is 0.2452 mol, because 25 ÷ 101.960 = 0.2452.
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.
Aluminum oxide is 101.960 g/mol, split 52.93% aluminum and 47.07% oxygen.
As corundum it is the second hardest common mineral after diamond, and trace impurities turn it into gemstones: chromium gives ruby, iron and titanium give sapphire. The bulk material, alumina, is the feedstock for aluminum smelting.
The mass ratio explains why aluminum was once more valuable than gold. Extracting the metal means breaking an extremely strong aluminum to oxygen bond, and every tonne of metal requires nearly two tonnes of oxide. Chemical reduction does not work, so the industry waited for cheap electricity and the Hall-Heroult process, and smelters are still sited next to hydroelectric dams for that reason.
A layer of this oxide a few nanometres thick forms on any exposed aluminum surface within seconds and is what stops the metal beneath from corroding further. Anodising deliberately thickens it.