For the equilibrium of the reaction:
NH4Cl(s)⇌NH3(g)+HCl(g);Kp=81atm2
Total pressure at equilibrium will be x times the pressure of NH3
The value of x will be 2.
Kp=PNH3PHCl=81
But PNH3=PHCl
Hence, Kp=P2NH3=81
PNH3=9=PHCl
Total pressure =PNH3+PHCl=9+9=18
Total pressure PNH3= 18 9 =2