Molar MassMolar mass of oxygen (O2)
The molar mass of oxygen, O2, is 32.00 g/mol. One mole of it, which is 6.022 × 1023 molecules, has a mass of 32.00 grams.
32.00 g/mol
2 × 15.999 = 31.998 · 2 atoms in the formula · also called dioxygen
About oxygen
Oxygen gas is made of molecules of two atoms. That is why its molar mass is twice the atomic weight of oxygen. Forgetting the 2 is the most common slip in mole sums about burning.
Other names: dioxygen.
The working
The formula O2 has 2 oxygen atoms. Multiply each count by the atomic weight of the element and add:
| Element | Atoms | Atomic weight | Total | Share by mass |
|---|---|---|---|---|
| Oxygen (O) | 2 | 15.999 | 31.998 | 100.00% |
2 × 15.999 = 31.998 g/mol
With the rounded atomic masses used in many school books (O = 16) the answer is 32 g/mol. The difference from 32.00 is too small to matter in most school sums.
Percentage composition
O2 contains only oxygen, so oxygen makes up 100% of its mass. Each molecule has 2 atoms, which is why the molar mass is 2 times the atomic weight 15.999.
Worked examples
Grams to moles. How many moles are in 5 g of O2? Divide the mass by the molar mass: 5 ÷ 32.00 = 0.1563 mol.
Moles to grams. What is the mass of 0.25 mol of O2? Multiply: 0.25 × 32.00 = 8 g.
Counting particles. 5 g contains 0.1563 mol × 6.022 × 1023 = 9.410 × 1022 molecules, and 1.882 × 1023 atoms in all.
As a gas. At 0 °C and 1 atmosphere one mole of an ideal gas occupies 22.4 litres, so the density of O2 is about 32.00 ÷ 22.4 = 1.43 g per litre. Air averages about 29 g/mol, so this gas is heavier than air.
Quick conversion tables
| Amount | Mass |
|---|---|
| 0.1 mol | 3.2 g |
| 0.5 mol | 16 g |
| 1 mol | 32 g |
| 2 mol | 64 g |
| 5 mol | 160 g |
| Mass | Moles |
|---|---|
| 1 g | 0.03125 mol |
| 10 g | 0.3125 mol |
| 100 g | 3.125 mol |
| 1 kg | 31.25 mol |
For other amounts use the grams and moles converter.
Questions people ask
What is the molar mass of O2?
The molar mass of oxygen (O2) is 32.00 g/mol, or 31.998 g/mol before rounding, using IUPAC abridged atomic weights.
How many moles are in 100 g of O2?
100 ÷ 32.00 = 3.125 mol.
What is the mass of one molecule of O2?
Divide the molar mass by the Avogadro constant: 32.00 ÷ 6.022 × 1023 = 5.313 × 10-23 g.
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.