The atomic mass of water is 18 grams per mole. Therefore, one liter of water contains 1000 grams * (1 mole / 18 grams) = 55.56 moles of water. Because there are 6.022 * 10^23 molecules in a mole, the number of molecules of water in a liter is (6.022 * 10^23) * 55.56 = approximately 3.345 * 10^25 molecules of water.