Molar Mass of Dioxygen

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.