Answer :
We know that Avogadro's number states:
1 mole H2O water = 18 g = 6.02x10^23 formula units (molecules, atoms, ions, etc.) ==> molecules of water here.
Procedure:
18 g water (1 mole) -------- 6.02x10^23 molecules of water
X --------- 1 molecule of water
X = 3.0x10^-23 g
Answer: 1 molecule of water = 3.0x10^-23 g