The weight of iron which will be converted into its oxide ($F{e}_3{O}_4$) by the action of 18g of steam on it will be (Atomic weight of Fe=56)

$3Fe+4H_{2}O\to F{e}_3{O}_4+4H_2$

          $3\times 56 4\times 18$
      =168        =72
     $\therefore$  72g steam required 168g of Fe
          $\frac{18}{72}\times 168=42g$ 18g steam required ____?