Molar Mass of Dioxygen

M(\mathrm{O_2}) = 0.031998\ \text{kg/mol}

Value	0.031998 kg/mol
Status	Measured: ± 0.000002 kg/mol (0.000063 relative)
Source	IUPAC
Categories	Chemistry molar-mass

M(O₂) in every molar mass unit

gram per mole	31.998 g/mol
kilogram per mole	0.031998 kg/mol

Learning zone

Oxygen gas is diatomic, so its molar mass is twice the atomic value: 31.998 g/mol, near enough 32. Combustion arithmetic runs straight off it — methane needs 2 mol of O₂ per mole of fuel, so 16.043 g of CH₄ requires 63.996 g of oxygen, and since air is 20.95 % O₂ by volume that means about 9.55 volumes of air per volume of methane.

The near-32 value is also why the old chemical atomic-mass scale was anchored to oxygen at exactly 16 until 1961, when physicists and chemists finally agreed to move to carbon-12 and end the embarrassment of two competing scales that differed by 275 parts per million. Dissolved oxygen work uses the same number in reverse: 8 mg/L of O₂ in cool river water is 0.25 mmol/L, and the difference between that and saturation is the deficit that drives reaeration.

Used by these solvers

Moles from Mass (n = m/M)

n = \frac{m}{M}

Ideal Gas Law

P V = n R T

Gas Density from Molar Mass

\rho = \frac{PM}{RT}