The mass of one mole of water it is 18 amu, but you need to find the mass of a molecule of water, therefore you calculate the mass of one mole of water, which is 18 amu and you divided by Avogadro's number which is 6,022 x 10^23. The result is 2,989 x 10^-23. Hope I helped you. If you have any questions ask :) Good luck.