According to the reaction we need 6 mols of water to produce 3 mols of H2 gas
but since they are asking about the mass we need to change these 3 mols to mass, for that we multiply the number of mols (3) by the molar mass of H2 (2):
[tex]\text{mass}=(nu\text{mber of mols )}\times molar\text{ mass= 3mols of H2}\times2\frac{grams\text{ of H2}}{\text{mols of H2}}=6\text{ grams of H2}[/tex]So after this calculation we know that 6 grams of H2 are produced each 6 mols of water are consumed using this as a conversion factor we can write the following equation:
[tex]600\text{ grams of H2}\times\frac{6\text{ mols of water}}{6\text{ grams of H2}}=600mols\text{ of water}[/tex]