Question: how many moles of molecules are there in 18 grams of hydrogen h2?
Answer: Hydrogen has a molecular mass of about 2 g/mole, so 18 g of hydrogen is 9 moles. One mole is 6.022 X 10^23 molecules, so the number of molecules in 18 g of hydrogen is 9X this, or approximately 54 X 10^23 = 5.4 X 10^24 molecules.