Answer:
x = 12.4 atm
Explanation:
step1: To calculate the pressure of NH3 in theflask using ideal gas equation      Â
       PV = nRT
Hence, we have
       P = 3.60 mol * 0.0821L.atm / mol.K * (450 +273)K /2.00L
         = 106.84 atm
step2: To calculate the equilibrium concentration of each component
        2NH3 ↔     N2   +     3H2
 initial(p)  106.84atm    0           0
change    -2x          +x         +3x
equilibrium (106.84-2x )    x          3x
Â
  k = PN2 PH23 /PNH32
6.3 = x * (3x)3 / (106.84-2x ) 2
since k is small , we ignore 2x compared to 106.84
otherwise to get the exact value of x u have to solve the equation above
  taking approx x = 12.4atm
then substitute the value of x in the respective equilibrium values
then find PT = PN2+ PH2+PNH3