the balanced equation for the formation of ammonia is as follows
N₂ + 3H₂ --> 2NH₃
stoichiometry of H₂ to NH₃ is 3:2
Avagadros law states that at volume of gas is proportional to number of moles of gas at contant pressure and temperature
therefore molar ratio is equivalent to volume ratio
hence, volume ratio of H₂ to NH₃ is 3:2
if 2 L of NH₃ requires 3 L of H₂
then 3.0 L of NH₃ requires - 3/2 x 3.0 L = 4.5 L of H₂
volume of H₂ required is
C.4.5L