Now, moles and a bogondo's number can be used to answer the following questions. For example how many molecules of water are in 20 grams of water? Now, to answer this question we have to first figure out how many moles are in 20 grams of water. And then we multiply that by our avocado's number remembering that 1 mol of anything is this many atoms or molecules in our case. Now, let's take our amount in grams of water and divide it by our molecular weight of water which we can find on the periodic table which is 18 grams/mol. The way we found that is we simply added up the atomic weight of oxygen plus two atomic weights of H because we have two H atoms.