If you know the mass of water lost, you can calculate using stoichiometry the moles of water. Oxygen has a molar mass of 16.00 grams per mole (g/mol) and hydrogen has a molar mass of 1.01 g/mol. In total, water has a molecular weight of 18.02 g/mol, so multiply the mass of your water lost by (1 mol / 18.02 g) because the grams need to cancel.