Can we vary the pressure while phases remain in equilibrium?

Consider a system containing only NH₄Cl(s), NH₃(g), and HCl(g).

Assume that the equilibrium NH₄Cl(s) ↔ NH₃g/ C HCl (g) exists.

(a) Suppose you prepare the system by placing solid NH₄Cl in an evacuated ?ask and heating to 400K. Use the phase rule to decide whether you can vary the pressure while both phases remain in equilibrium at 400K.

(b) According to the phase rule, if the system is not prepared as described in part (a) could you vary the pressure while both phases remain in equilibrium at 400K? Explain.

(c) Rationalize your conclusions for these two cases on the basis of the thermodynamic equilibrium constant. Assume that the gas phase is an ideal gas mixture and use the approximate expression K D pNH₃ pHCl =(p°)2.