The equation for the synthesis of ammonia is:
N₂ + 3 H₂ ---> 2 NH₃
We have to find the number of moles of H₂ needed to produce 6 mol of NH₃. To do that we will use the equation that we were given.
We can read the equation of the reaction as a recipe. When 1 mol of N₂ reacts with 3 moles of H₂, 2 moles of NH₃ are produced. So the relationship between H₂ and NH₃ is that 3 moles of H₂ are needed to produce 2 moles of NH₃. We wiill use that relationship to solve the problem.
6 moles of NH₃ * 3 moles of H₂/(2 moles of NH₃) = 9 moles of H₂
Answer: D) 9 moles of H₂ are needed.