So 16 for oxygen and two times one for H two gives us 18 grams/mol for a single water molecule. So 20 grams of water divided by 18 grams/mol we see that the grams cancel, the moles go on top and we get one point moles of water. So this means within our 20 grams of water there are one point eleven moles of water. Now, we know that in 1 mol of anything there are this many molecules. So to find the number of molecules in one point eleven moles of water we simply multiply this number by Avogastro's number. So we get 6.22 times ten to the 23 atoms.