there are two moles of H in H2O. you have ten moles of H2 gas (presumably), meaning 20 moles of H, so you could (would) make 10 moles of H2O (use 20 moles of H) in that situation. Just figure out the molar mass of H2O (roughly 18, but not precisely so) and multiply by 10.

Learn how to do this basic thing, because you will have to revisit the idea many times, of converting moles to mass and mass to moles, and figuring out stoichiometry of reactions.

The coefficients in the balanced equation determine the ratio of the moles of reactants to moles of products. In this equation, the coefficient for Hydrogen gas is the same as the coefficient for water. This means 10 moles of water is produced. The mass of one mole of water is 18 grams. So 180 grams of water is produced. I hope this is helpful for you.