The molar mass of oxygen gas (O2) is 31.998 g/mol.
Case matters: CO is carbon monoxide, Co is
cobalt. Brackets and hydrates are fine — try
or
.
| Element | Atoms | Atomic mass | Mass contributed | Percent |
|---|---|---|---|---|
| Oxygen (O) | 2 | 15.999 | 31.998 | 100.00% |
| Total | 2 | 31.998 | 100.00% |
31.998 = 31.998 g/mol
Grams and moles
25 g of oxygen gas is 0.7813 mol, because 25 ÷ 31.998 = 0.7813.
This element has no single atomic weight. Their isotopic mix varies slightly depending on where on Earth the sample came from, so the figure used here is the conventional value IUPAC publishes for when one number is needed: oxygen is 15.999, and really lies between 15.99903 and 15.99977.
Oxygen in air is not loose atoms. It is O2, a diatomic molecule, and its molar mass is 31.998 g/mol rather than the 15.999 of a single oxygen atom. Getting those two confused doubles or halves an answer, and it is one of the most common errors in gas stoichiometry.
The distinction matters whenever a balanced equation is involved. Burning methane consumes two O2 molecules, which is 63.996 grams of oxygen gas, not four atoms weighed separately. Reaction arithmetic counts molecules of the substance as it actually exists.
A mole of it fills roughly 24 litres under ordinary laboratory conditions, so those 32 grams take up about as much space as a household bucket. Air is about 21% oxygen by volume, which works out slightly higher by mass at around 23%, because O2 is a little heavier than the nitrogen making up most of the rest.
Ozone, O3, is the same element again at 47.997 g/mol and behaves nothing like it.