Answer:
9 moles of N2
Explanation:
Well first you balance the equation which would be N2+3 H2>2 NH3. But you need 18 miles of NH3 so you would just replace the 2 moles of NH3 with 18 and balance the rest of the equation from there. And you would get 9 N2 + 27 H2>18 NH3
