Question: how many water molecules in a gallon?
Answer: The density of water is 1g/(cm3) so in 1 gallon of water ( about 3.785 Liters or 3785 cm3) the mass of the water is, 3785g. 1 mole of 6.02x1023 molecules of water is equal has the mass in grams equal to the molecular weight or 18 grams per mole. so 3785 grams corresponds to about 1.265 x 1026 molecules of water.