The molar mass of magnesium oxide (MgO) is 40.304 g/mol.
Case matters: CO is carbon monoxide, Co is
cobalt. Brackets and hydrates are fine — try
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| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 60.30% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 39.70% |
| Total | 2 | 40.304 | 100.00% |
24.305 + 15.999 = 40.304 g/mol
Grams and moles
25 g of magnesium oxide is 0.6203 mol, because 25 ÷ 40.304 = 0.6203.
These elements have 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: magnesium is 24.305, and really lies between 24.304 and 24.307; oxygen is 15.999, and really lies between 15.99903 and 15.99977.
Magnesium oxide is 40.304 g/mol, made of magnesium at 60.30% and oxygen at 39.70%.
It is what remains after burning magnesium ribbon, and the mass gain in that reaction is the standard school experiment for establishing a formula by experiment rather than by assertion. Weigh the ribbon, burn it, weigh the white ash, and the ratio of magnesium to oxygen comes out near 24.305 to 15.999. That is a one to one atom ratio, which gives MgO rather than MgO2 or Mg2O.
The experiment rarely gives a clean answer, and the reasons are worth knowing: some magnesium nitride forms because magnesium is reactive enough to burn in nitrogen, and some oxide smoke escapes the crucible. Both push the result in predictable directions.
Its melting point above 2,800 degrees Celsius makes it a standard refractory lining for furnaces.