Wouldn't that change though based upon the isotopes of the atoms in it? For example H2O made with Deuterium (1 proton 1 neutron and 1 electron) or Tritium (1 proton 2 neutrons and one electron) vs Protium (1 proton 1 neutrons and 1 electron), could all give different weights, not to mention different isotopes of oxygen 16 17 and 18 for the most common/long lived.