Question: how many atoms of hydrogen are there in 18 g of water?
Answer: Water has a molar mass of 18.01528 g/mol≈18 g/mol . Since water has a chemical formula of H2O , there will be 2 moles of hydrogen in every mole of water. In one mole of water, there will exist approximately 6.02⋅1023 water molecules. So, there will be a total of 6.02⋅1023⋅2≈1.2⋅1024 hydrogen atoms.