Molar MassMolar mass of magnesium oxide (MgO)
The molar mass of magnesium oxide, MgO, is 40.30 g/mol. One mole of it, which is 6.022 × 1023 formula units, has a mass of 40.30 grams.
40.30 g/mol
24.305 + 15.999 = 40.304 · 2 atoms in the formula · also called magnesia
About magnesium oxide
Magnesium oxide is the white ash left when magnesium ribbon burns. Finding its formula by weighing the ribbon before and the ash after is a standard school experiment on empirical formulas.
Other names: magnesia.
The working
The formula MgO has 1 magnesium atom and 1 oxygen atom, 2 atoms in all. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Magnesium (Mg) | 1 | 24.305 | 24.305 | 60.30% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 39.70% |
24.305 + 15.999 = 40.304 g/mol
With the rounded atomic masses used in many school books (Mg = 24, O = 16) the answer is 40 g/mol. The difference from 40.30 is small, but it shows why you should use the values your question paper gives.
Percentage composition
By mass, MgO is 60.30% magnesium and 39.70% oxygen. The largest share comes from magnesium. In a 100 g sample there are 60.3 g of magnesium and 39.7 g of oxygen.
Worked examples
Grams to moles. How many moles are in 50 g of MgO? Divide the mass by the molar mass: 50 ÷ 40.30 = 1.241 mol.
Moles to grams. What is the mass of 0.4 mol of MgO? Multiply: 0.4 × 40.30 = 16.12 g.
Counting particles. 50 g contains 1.241 mol × 6.022 × 1023 = 7.471 × 1023 formula units.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 4.03 g |
| 0.5 mol | 20.15 g |
| 1 mol | 40.3 g |
| 2 mol | 80.61 g |
| 5 mol | 201.5 g |
| Mass | Moles |
|---|---|
| 1 g | 0.02481 mol |
| 10 g | 0.2481 mol |
| 100 g | 2.481 mol |
| 1 kg | 24.81 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of MgO?
The molar mass of magnesium oxide (MgO) is 40.30 g/mol, or 40.304 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of MgO?
100 ÷ 40.30 = 2.481 mol.
What is the mass of one formula unit of MgO?
Divide the molar mass by the Avogadro constant: 40.30 ÷ 6.022 × 1023 = 6.693 × 10-23 g.
What is the percentage of magnesium in MgO?
Magnesium makes up 60.30% of the mass of MgO: 1 × 24.305 ÷ 40.304 × 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.