Molar MassMolar mass of oxides
An oxide is a compound of oxygen with one other element. Metal oxides are mostly basic solids, non-metal oxides are mostly acidic gases or liquids.
Oxides appear in the first mole calculations a student meets: burning magnesium, heating limestone, reducing copper oxide. The percentage of metal in an oxide ore, such as iron in haematite, is a standard question. Roman numerals in a name, as in iron(III) oxide, tell you the charge on the metal and so fix the formula.
This page lists 22 oxides with their formulas and molar masses, lightest first. The lightest here is water at 18.02 g/mol and the heaviest is iron(II,III) oxide at 231.53 g/mol. Tap a name for the full working, the percentage of each element and worked examples.
| Compound | Formula | Molar mass, g/mol |
|---|---|---|
| Water dihydrogen monoxide | H2O | 18.02 |
| Carbon monoxide | CO | 28.01 |
| Nitrogen monoxide nitric oxide | NO | 30.01 |
| Hydrogen peroxide | H2O2 | 34.01 |
| Magnesium oxide magnesia | MgO | 40.30 |
| Carbon dioxide | CO2 | 44.01 |
| Dinitrogen monoxide nitrous oxide | N2O | 44.01 |
| Nitrogen dioxide | NO2 | 46.01 |
| Calcium oxide quicklime | CaO | 56.08 |
| Silicon dioxide silica | SiO2 | 60.08 |
| Sodium oxide | Na2O | 61.98 |
| Sulphur dioxide sulfur dioxide | SO2 | 64.06 |
| Copper(II) oxide cupric oxide | CuO | 79.55 |
| Titanium dioxide titania | TiO2 | 79.86 |
| Sulphur trioxide sulfur trioxide | SO3 | 80.06 |
| Zinc oxide | ZnO | 81.38 |
| Manganese(IV) oxide manganese dioxide | MnO2 | 86.94 |
| Aluminium oxide alumina | Al2O3 | 101.96 |
| Phosphorus pentoxide phosphorus(V) oxide | P2O5 | 141.94 |
| Iron(III) oxide ferric oxide | Fe2O3 | 159.69 |
| Lead(II) oxide litharge | PbO | 223.20 |
| Iron(II,III) oxide magnetite | Fe3O4 | 231.53 |
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.
Not in the list?
Type any formula into the calculator. It handles brackets and water of crystallisation and shows the same working as these pages. The method by hand is on the step by step page.
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