To completely answer this question, the reaction is needed. However, I can show you how to set the equation up. The most important thing that you will need from the BALANCED chemical equation is the molar ratio between H2 and H2O. The molar ratio will consist of the coefficients in front of the two compounds.
This is how you would set up the problem. Start with the number of moles of H2 multilply by the molar ratio from the balanced equation and then multiply by the molar mass of water (18 g / mol) to get the answer of grams of H2O.