How many Hydrogen atoms are there in 500.0 g of H20?

The chemical formula of water is H2O, showing that each molecule contains two hydrogen atoms. The gram molecular mass of water is 18.016. Therefore, 52.0 grams of water contains 52.0/18.016 or about 2.886 moles of water.

Avogadro's number is defined as the number of molecules per gram mole and is 6.023 X 1023 atoms, molecules or ions. Therefore, 2.866 moles contains:

2 X 2.866 X 6.023 X 1023 or 3.45 X 1024 hydrogen atoms, to the justified number of significant digits.