This is a stoichiometry problem, where we have an initial amount of reactant and we need to find out how much of the product will we end up with, in order to do that we need to:
1. Set up the properly balanced equation, which the question already provided us
2 H2 + O2 -> 2 H2O
2. See how many moles of reactant there are in the given amount of grams, which we don't have to do since the question already gave us the number of moles of Hydrogen, 6.0 moles
3. Check the molar ratio between the two compounds, which will be 2:2, the same amount of moles of H2 is needed to produce the same amount of moles of H2O, therefore if we have 6 moles of H2, we will also have 6 moles of H2O
4. Calculate how many grams will be equal to the number of moles that we found out, we will do that by calculating the number of moles with the molar mass, 18g/mol
18g = 1 mol
x grams = 6 moles
x = 108 grams of H2O will be produced
For the purpose of the table, everything I write left will be on the top side, everything I write right will be on the bottom
6 moles of H2
2 moles of H2O/2 moles of H2 (H2 will cancel out)
18 grams of H2O/1 mol of H2O (mol will cancel)
108 grams of H2O will be the final answer