Question: how many atoms of hydrogen are present in 7.85 g of water?
Answer: 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.