Molar mass of oxygen (O₂)

The molar mass of oxygen (O₂) is 31.998 g/mol, summed from the IUPAC standard atomic weights of O x 2. Molecular oxygen, not the atomic weight of the element.

Mass of sample

Amount of substance

mol

Molar mass

31.998g/mol

= Exact Molar mass of oxygen

Moles in the sample mass: 0.312 519 53 mol
Mass of that amount of substance: 31.998
Molecules in the sample mass: 1.88204e+23

Show the working

Mass of sample = 10
Amount of substance = 1
Molar mass = 15.999*2 = 31.998
Moles in the sample mass = 10 / 31.998 = 0.312 52
Mass of that amount of substance = 1 * 31.998 = 31.998
Molecules in the sample mass = 0.312 52 * NA = 1.88204e+23

Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.

Scenarios & local history

Compare a saved scenario

Recent calculations

The formula

Molar mass = sum over elements of (atomic weight x number of atoms)
For O₂: 15.999 x 2 = 31.998 g/mol
Moles = mass / molar mass; mass = moles x molar mass
Molecules = moles x 6.02214076 x 10^23 (the Avogadro constant, exact by definition)

Molar mass in grams per mole is numerically equal to the molecular mass in daltons, so this figure doubles as the molecular weight.

Atomic weights are the IUPAC 2021 standard values. Several elements have a range rather than a single value because isotope ratios vary between sources; the conventional single value is used here.

Molecular oxygen, not the atomic weight of the element.

Composition by mass

Element	Atoms	Atomic weight	Contribution (g/mol)	Share of mass
Oxygen (O)	2	15.999	31.998	100 %

Contributions add to the molar mass of 31.998 g/mol. Atomic weights are the IUPAC 2021 standard values.

Frequently asked

What is the molar mass of oxygen?

31.998 g/mol, for the formula O₂.

How do you convert grams of oxygen to moles?

Divide the mass in grams by 31.998 g/mol. One gram of oxygen is 0.031252 mol.

How many grams are in one mole of oxygen?

31.998 g, by definition of the molar mass.

Is molar mass the same as molecular weight?

Numerically yes, for the purposes of this calculation. Molecular weight is a dimensionless ratio and molar mass carries units of g/mol, but the digits are identical.

Where the numbers come from

IUPAC 2021 standard atomic weights; Avogadro constant fixed by the 2019 SI.