The molar mass of oxygen, O2, is 31.998 g/mol. That single number is what connects a mass you can weigh to an amount of substance you can react: one mole is 31.998 g, and a gram of it is 1.882e+22 formula units. The calculator turns any weighed mass into that count, and reports the amount in moles, millimoles and the mass of a single formula unit alongside it.
The figure comes from the formula. Oxygen is 2 × 15.999 (O), which is 31.998 g/mol, taking standard atomic weights, and every page here computes that sum rather than quoting it. Oxygen makes up the largest share of the mass: 2 of the 2 atoms in a formula unit are oxygen, and they account for 31.998 g of the 31.998 g, or 100% by mass. The derivation table below breaks the whole molecule down element by element.
The numbers a bench actually uses sit at the millimole scale. One millimole of oxygen is 31.998 mg and one micromole is 31.998 µg, so a balance reading to a milligram places you within 31.25 µmol. A single formula unit weighs 5.3134e-23 g, which is why the count in the gauge runs into the sextillions for any mass you can see.
Diatomic, which is the detail that catches people out: the molar mass is twice the atomic weight, and stoichiometry needs the molecule. At 31.998 g/mol it is heavier than 5 of the 11 gases and small molecules covered here, and that ranking matters more than it looks: nitrogen has a molar mass of 28.014 g/mol, so a gram of it contains 14.2% more formula units than a gram of oxygen. Weigh by mass and you are not weighing equal amounts of substance.
Because this is a gas, volume is often the easier measure. One mole of any ideal gas occupies 22.414 L at 0 °C and 1 atm and 24.4655 L at 25 °C, so 31.998 g of oxygen — one mole — fills about 24.47 litres at room temperature. Real gases depart from that by a percent or so, more near their boiling point, but for ordinary work the molar volume is the quickest route from grams to litres.
The formula
m- The mass of oxygen in grams
31.998- The molar mass of oxygen in g/mol, derived from its formula
N- The number of formula units that mass contains
How it works, step by step
- Enter a mass of oxygen in grams.
- It is divided by the molar mass, 31.998 g/mol, giving the amount in moles.
- That amount is multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole, to give the number of formula units.
- The panel also reports the amount in moles and millimoles and the mass of one formula unit.
Worked examples
0.5 g of oxygen as a molecule count
0.5 g divided by 31.998 g/mol is 0.015626 mol, and multiplying by the Avogadro constant gives 9.41e+21 formula units of oxygen. The same mass is 15.626 mmol, which is the figure a reaction is actually planned in.
5 g of oxygen as a molecule count
5 g divided by 31.998 g/mol is 0.15626 mol, and multiplying by the Avogadro constant gives 9.41e+22 formula units of oxygen. The same mass is 156.26 mmol, which is the figure a reaction is actually planned in.
50 g of oxygen as a molecule count
50 g divided by 31.998 g/mol is 1.5626 mol, and multiplying by the Avogadro constant gives 9.41e+23 formula units of oxygen. The same mass is 1562.6 mmol, which is the figure a reaction is actually planned in.
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Frequently asked questions
What is the molar mass of oxygen (O2)?
It is 31.998 g/mol. That is the sum of the standard atomic weights of every atom in the formula: 2 × 15.999 (O). One mole of oxygen therefore weighs 31.998 g, and a gram of it is 31.252 mmol.
What percentage of oxygen is oxygen?
100% by mass. Each formula unit contains 2 oxygen atoms contributing 31.998 g of the 31.998 g total, so 100 g of oxygen contains 100 g of oxygen and a kilogram contains 1000 g of it.
How many molecules are in one gram of oxygen?
About 1.882e+22. One gram is 0.031252 mol, and each mole contains 6.02214076 × 10²³ formula units by definition of the mole, so the count is that constant divided by 31.998.
How much does a single molecule of oxygen weigh?
5.3134e-23 g, which is 31.998 daltons. It is the molar mass divided by the Avogadro constant, and the number in daltons is numerically the same as the molar mass in g/mol — that equivalence is what makes the mole convenient.
Is molar mass the same as molecular weight?
They are the same number in ordinary use but not the same quantity. Molecular weight, properly relative molecular mass, is a ratio and has no unit: for oxygen it is 31.998. Molar mass carries grams per mole: 31.998 g/mol. Because the mole is defined so those agree, you can read one off the other.
Why is the molar mass not a whole number?
Because standard atomic weights are averages over the isotopes found in nature. Oxygen is quoted as 15.999 rather than a whole number for that reason, and the same applies to the other elements here, which is why oxygen comes out at 31.998 g/mol rather than 32.
Molar mass, composition and mole equivalents of oxygen
| Element | Atoms | Atomic weight | Contribution (g/mol) | By mass |
|---|---|---|---|---|
| Oxygen (O) | 2 | 15.999 | 31.998 | 100% |
| Total — one mole of oxygen | 31.998 | 100% |
Standard atomic weights, IUPAC 2021. The contribution column is atoms × atomic weight, and the total is the molar mass this page uses: 31.998 g/mol.
| Amount | Mass (g) | Mass (mg) | Formula units |
|---|---|---|---|
| 1 µmol | 0.000032 | 0.031998 | 6.022e+17 |
| 10 µmol | 0.00031998 | 0.31998 | 6.022e+18 |
| 100 µmol | 0.0031998 | 3.1998 | 6.022e+19 |
| 1 mmol | 0.031998 | 31.998 | 6.022e+20 |
| 10 mmol | 0.31998 | 319.98 | 6.022e+21 |
| 50 mmol | 1.5999 | 1599.9 | 3.011e+22 |
| 100 mmol | 3.1998 | 3199.8 | 6.022e+22 |
| 250 mmol | 7.9995 | 7999.5 | 1.506e+23 |
| 500 mmol | 15.999 | 15,999 | 3.011e+23 |
| 750 mmol | 23.9985 | 23,998.5 | 4.517e+23 |
| 1 mol | 31.998 | 31,998 | 6.022e+23 |
| 2 mol | 63.996 | 63,996 | 1.204e+24 |
| 5 mol | 159.99 | 159,990 | 3.011e+24 |
| 10 mol | 319.98 | 319,980 | 6.022e+24 |
Mass is the amount multiplied by 31.998 g/mol. The last column is that amount multiplied by the Avogadro constant, 6.02214076 × 10²³ per mole.
| Compound | Formula | Molar mass (g/mol) | Millimoles in 1 g |
|---|---|---|---|
| Water | H2O | 18.015 | 55.509 |
| Carbon monoxide | CO | 28.01 | 35.702 |
| Nitrogen | N2 | 28.014 | 35.696 |
| Oxygen (this page) | O2 | 31.998 | 31.252 |
| Hydrogen peroxide | H2O2 | 34.014 | 29.4 |
| Carbon dioxide | CO2 | 44.009 | 22.723 |
| Propane | C3H8 | 44.097 | 22.677 |
Ordered by molar mass. The last column is 1000/M, which is the number a weighed gram actually gives you.