Going from grams to moles for oxygen means dividing by its molar mass, 31.998 g/mol. One gram is 0.031252 mol, or 31.252 mmol; 100 mg is 3.1252 mmol; and a 31.998 g portion is exactly one mole. The division is the whole calculation, but the molar mass has to be the one for the exact formula you have, O2, which is where these figures come from.
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.
Balance resolution is the practical limit here. On a two-decimal balance the smallest step is 10 mg, which is 0.3125 mmol of oxygen; on a three-decimal balance it is 1 mg, or 31.25 µmol. To hit a target within 1% you therefore need to weigh at least a gram on the first and at least 100 mg on the second, and below that a stock solution measured by pipette beats weighing.
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 you weighed, in grams
31.998- The molar mass of oxygen in g/mol, derived from its formula
n- The amount of substance in moles
How it works, step by step
- Enter the mass you weighed, in grams.
- It is divided by 31.998 g/mol, the molar mass of oxygen.
- The result is the amount of substance in moles, with millimoles and micromoles beneath it.
- Check it against the table if you want the figure for a standard weighing rather than a typed mass.
Worked examples
0.25 g of oxygen in moles
0.25 ÷ 31.998 = 0.00781299 mol, or 7.813 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 31.25 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
2 g of oxygen in moles
2 ÷ 31.998 = 0.0625039 mol, or 62.504 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 31.25 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
25 g of oxygen in moles
25 ÷ 31.998 = 0.781299 mol, or 781.3 mmol. If the balance was reading to 1 mg, the last digit of that mass is worth 31.25 µmol, so quoting the answer to more than four significant figures claims a precision the weighing did not have.
How to read your score
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 do I convert grams of oxygen to moles?
Divide the mass in grams by 31.998 g/mol. So 1 g is 0.031252 mol, 10 g is 0.31252 mol and 100 g is 3.1252 mol. Nothing else enters the calculation — no volume, no temperature, no concentration.
How many moles is 100 g of oxygen?
3.1252 mol, which is 3125.2 mmol. Read the other way, one mole of oxygen is 31.998 g, so 100 g is 3.125 of a mole.
What about milligrams and micromoles?
The same division, scaled. A milligram of oxygen is 31.252 µmol, 10 mg is 312.52 µmol and 100 mg is 3.1252 mmol. Working in µmol per mg avoids the string of leading zeros that mol per g produces at this scale.
Does purity change the number of moles?
Yes, in proportion. A 98% pure sample weighing 1 g contains 0.98 g of oxygen, which is 0.0306269 mol rather than 0.031252 mol. The calculator assumes the mass you enter is the compound itself, so divide by the assay figure first if you need the corrected amount.
Grams to moles reference for 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.
| On the balance | Moles | Millimoles | Micromoles |
|---|---|---|---|
| 1 mg | 0.000031 | 0.031252 | 31.252 |
| 5 mg | 0.00015626 | 0.15626 | 156.26 |
| 10 mg | 0.00031252 | 0.31252 | 312.52 |
| 25 mg | 0.000781299 | 0.781299 | 781.299 |
| 50 mg | 0.0015626 | 1.5626 | 1562.6 |
| 100 mg | 0.0031252 | 3.1252 | 3125.2 |
| 250 mg | 0.00781299 | 7.81299 | 7812.99 |
| 500 mg | 0.015626 | 15.626 | 15,626 |
| 1 g | 0.031252 | 31.252 | 31,252 |
| 2 g | 0.0625039 | 62.5039 | 62,503.9 |
| 5 g | 0.15626 | 156.26 | 156,260 |
| 10 g | 0.31252 | 312.52 | 312,520 |
| 25 g | 0.781299 | 781.299 | 781,299 |
| 50 g | 1.5626 | 1562.6 | 1562597.662354 |
| 100 g | 3.1252 | 3125.2 | 3125195.324708 |
| 250 g | 7.81299 | 7812.99 | 7812988.311769 |
| 500 g | 15.626 | 15,626 | 15625976.623539 |
Every row is the mass divided by 31.998 g/mol. A balance reading to 1 mg resolves 31.25 µmol of this compound, which is the smallest step the middle columns can really move in.
| 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.