To calculate the molecular weight of water, we simply add the atomic weight of oxygen plus two times the atomic weight of H because we have a substrate of two, so we get 16 grams/mol of oxygen plus two times 1 gram/mol of H gives us 18 grams/mol. So the molecular weight of water is 18 grams/mol. Now, to find the amount in grams of water that we have in our initial solution, we need to multiply the molecular weight by the 25 moles of H 20 that we have. So 18 grams/mol times 25 moles gives you 450. Now, moles cancel, the grams are left, so we have 450 grams of H 20. Now, we want to find the amount of HCL we need to add in terms of grams to create a 90,000 parts per million solution.