Moles to grams for oxygen is a multiplication by 31.998 g/mol. A mole weighs 31.998 g, 100 mmol weighs 3.1998 g, 10 mmol weighs 0.31998 g and a millimole is 31.998 mg. Those four figures cover most of what gets weighed out in practice, and the table below extends them from a micromole up to ten moles.
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.
Two habits make the weighed mass match the calculation. Weigh into a container you have tared rather than onto paper, because what stays behind on paper is a real loss — on a 100 mg weighing it is easily a percent. And when the target is under about 20 mg, weigh ten times that and dilute, since the absolute error of the balance does not shrink with the sample.
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
n- The amount of oxygen you need, in moles
31.998- The molar mass of oxygen in g/mol, derived from its formula
m- The mass to weigh out, in grams
How it works, step by step
- Enter the amount you need, in moles.
- It is multiplied by 31.998 g/mol.
- The result is the mass to weigh out, also given in milligrams.
- Weigh into a tared container; anything left in the boat is a real loss from the amount you calculated.
Worked examples
5 mmol of oxygen in grams
5 mmol × 31.998 g/mol = 0.15999 g, which is 159.99 mg. Doubling the amount doubles the mass to 0.31998 g, because the relationship is a plain proportion once the molar mass is fixed.
250 mmol of oxygen in grams
250 mmol × 31.998 g/mol = 7.9995 g, which is 7999.5 mg. Doubling the amount doubles the mass to 15.999 g, because the relationship is a plain proportion once the molar mass is fixed.
2 mol of oxygen in grams
2 mol × 31.998 g/mol = 63.996 g, which is 63,996 mg. Doubling the amount doubles the mass to 127.992 g, because the relationship is a plain proportion once the molar mass is fixed.
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 many grams is one mole of oxygen?
31.998 g, by definition of the molar mass. Two moles is 63.996 g and half a mole is 15.999 g.
How many grams is 0.1 mol, and how many milligrams is 1 mmol?
0.1 mol of oxygen is 3.1998 g. One millimole is 31.998 mg and one micromole is 31.998 µg, which is the range most bench work sits in.
How do I weigh out an amount too small for my balance?
Weigh a larger mass and dilute it. To get 0.32 mg of oxygen, weigh 3.2 mg into 100 mL, then take 10 mL of that: the absolute error of the balance stays the same while the amount delivered falls by a factor of ten.
Should I use the hydrate or the anhydrous molar mass?
This page is the anhydrous form, O2, at 31.998 g/mol. If your jar is the other form the molar mass differs and so will every mass you weigh, so check the label before using the figure. Salts that are sold as hydrates weigh more per mole because the water is part of the crystal.
Moles to grams 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.
| Wanted | Weigh (g) | Weigh (mg) | With 2% over (g) |
|---|---|---|---|
| 1 µmol | 0.000032 | 0.031998 | 0.000033 |
| 10 µmol | 0.00031998 | 0.31998 | 0.00032638 |
| 100 µmol | 0.0031998 | 3.1998 | 0.0032638 |
| 1 mmol | 0.031998 | 31.998 | 0.032638 |
| 10 mmol | 0.31998 | 319.98 | 0.32638 |
| 50 mmol | 1.5999 | 1599.9 | 1.6319 |
| 100 mmol | 3.1998 | 3199.8 | 3.2638 |
| 250 mmol | 7.9995 | 7999.5 | 8.15949 |
| 500 mmol | 15.999 | 15,999 | 16.319 |
| 750 mmol | 23.9985 | 23,998.5 | 24.4785 |
| 1 mol | 31.998 | 31,998 | 32.638 |
| 2 mol | 63.996 | 63,996 | 65.2759 |
| 5 mol | 159.99 | 159,990 | 163.19 |
| 10 mol | 319.98 | 319,980 | 326.38 |
Weigh is the amount multiplied by 31.998 g/mol. The last column carries a 2% allowance, which is what you take when some of the solid will be left in the weighing boat.
| 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.