To determine the number of molecules of H2O that is being produced, we need to convert moles to molecules. We need to use a conversion factor. For this case, we use Avogadro's number. It represents the number of units in one mole of any substance. This has the value of 6.022 x 10^23 units / mole. This number can be used to convert the number of atoms or molecules into number of moles. We calculate as follows:
0.819 moles H2O ( 6.022 x 10^23 molecules / 1 mole) = 4.93x10^23 molecules
The number of molecules of H2O produced would be 4.93x10^23 molecules.