First, find how many grams are in 1 mole of water.
For a hydrogen atom, there is about 1 gram per mole. For an oxygen atom, there are about 16 grams per mole.
In H2O, there are two hydrogen atoms and one oxygen atom. This means there are 18 grams in one mole of water. Multiply the mass in one mole by your number of moles.
