A nitrogen-hydrogen mixture initially in the molar ratio of 1 : 3 reached equilibrium to form ammonia when 25% of the N2 and H2 had reacted. If the total pressure of the system was 21 atm, the partial pressure of ammonia at the equilibrium was:
(a) 4.5 atm
(b) 3.0 atm
(c) 2.0 atm
(d) 1.5 atm
