If you start out with 9 moles of Fe, how many moles of H2O will you need to react it completely?

13.5 moles of H₂O

Explanation:

We determine the reaction

2Fe + 3H₂O → Fe₂O₃ + 3H₂

2 moles of iron react with 3 moles of water, in order to produce 3 moles of hydrogen and 1 mol of iron (III) oxide.

Ratio is 2:3, so let's make a rule of three:

2 moles of iron need 3 moles of water to react

Then, 9 moles of iron will react with (9 . 3) /2 = 13.5 moles of H₂O