Answer is: three. Unbalanced chemical reaction: Fe+4H₂O → Fe₃O₄ + 4H₂. Balanced chemical reaction: 3Fe+4H₂O → Fe₃O₄ + 4H₂. In balanced reaction, there is three iron atoms on the left and on the right, eight hydrogen atoms and four oxygen atoms on both side of chemical reaction.
This question is about stoichiometry. From the balanced equation ⇒, we see that 3 moles of water is needed to react with 1 mole of .
This means that, to fully react 3.62 moles of , we would need 3*3.62 or 10.86 moles of water. However, we only have 6.31 moles, so water is the limiting reactant.
Since 3 moles of water react with 1 mole of , 6.31 moles of water can fully react with 6.31÷3 or 2.1033 moles of .
From the balanced equation, we see that every mole of reacted gets you 2 moles of . Therefore, 2.1033 moles of would give you approximately 4.21 moles of .