The molar mass of water (H2O) is 18.015 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 |
|---|---|---|---|---|
| Hydrogen (H) | 2 | 1.0080 | 2.016 | 11.19% |
| Oxygen (O) | 1 | 15.999 | 15.999 | 88.81% |
| Total | 3 | 18.015 | 100.00% |
2.016 + 15.999 = 18.015 g/mol
Grams and moles
25 g of water is 1.388 mol, because 25 ÷ 18.015 = 1.388.
These elements have 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: hydrogen is 1.0080, and really lies between 1.00784 and 1.00811; oxygen is 15.999, and really lies between 15.99903 and 15.99977.
Water is the reference substance for almost everything in introductory chemistry, and its molar mass is the first one most students commit to memory. Two hydrogens contribute 2.016 g/mol between them and the single oxygen contributes 15.999, which is why oxygen accounts for 88.81% of the mass of water while making up only a third of its atoms. That gap between atom count and mass share is the thing worth taking away from this page.
A mole of water is 18.015 grams, and because water has a density close to 1 g/mL that is just over 18 millilitres, roughly a tablespoon. Very few substances let you picture a mole so directly, which is why water is the example every course starts with.
Do not confuse this with the mass of a single water molecule, which is about 2.99 x 10-23 grams. The 18.015 figure applies to 6.022 x 1023 of them at once, and that is the entire reason the mole exists as a unit.