Divide the mass in grams of H2O by the molecular weight of H2O in grams per mole. That gives you the number of moles present. Multiply that by Avogadro's number to get the number of molecules present.
n = number of moles
molecular weight of water ≅ 18.0 grams /mole
n = (150.0 grams) / (18.0 grams / mole) = 8.33 moles
N = number of molecules
N = (6.02 x 1023)(8.33) = 5.01 x 1024 molecules