Molar Mass of Water

Water's molar mass is the most-used number in chemistry after the mole itself: 2 × 1.008 for the hydrogens plus 15.999 for the oxygen gives 18.015 g/mol. Because liquid water's density is close to 1.00 g/mL, one mole is about 18 mL, and a litre of water is 55.5 mol — the number that quietly sets the reference state for every dilute-solution activity and the reason the solvent is left out of an equilibrium expression.

The value carries a real uncertainty, ±0.001 g/mol, because both hydrogen and oxygen have isotopic compositions that vary with source. Antarctic snowmelt is measurably lighter than Mediterranean seawater; heavy water, D₂O, has a molar mass of 20.028 g/mol and freezes at 3.8 °C rather than 0 °C. For laboratory work the standard value is far more precise than any balance you will use, but for isotope-ratio mass spectrometry the variation is the whole signal.