How many atoms of H are in 7.1 moles of H2O?

Answer 1

7.1 moles Water * (2 moles H/mole W) * (6.022 x 10²³ atoms H/mole H) = atoms H

Use units to cancel to get from known quantity to quantity you are looking for. Put in numbers that make the multipliers true (= in the case of conversions). There are 2 moles H/mole W due to formula stoichiometry. There are Avogadro's Number of atoms in a mole of atoms.