18 grams

The mass of one mole of any atom/molecule is equal to its atomic molecular mass in grams.

The molecular formula of water is ##H_2O##

The atomic mass of ##H## = 1

The atomic mass of ##O## = 16

In a sense, the formula means ##H## + ##H## + ##O##

So… 1 + 1 + 16 = 18

Therefore, the mass of one mole of water = 18 Grams