A vessel contains 1 mole PCl_{5 (g)} at 4 atm and 0.5 mole PCl₃ formed at equilibrium. Now, equilibrium pressure of mixture is (assume ideal behavior)

p $\infty$n (V,T are constant)

initial moles = 1 mole 

at equilibrium total moles = (1-0.5)+0.5+0.5 = 1.5

$$\begin{gathered} \frac{{\mathrm{p}}_{\mathrm{initial}}}{{\mathrm{p}}_{\mathrm{final} }}=\frac{{\mathrm{n}}_{\mathrm{initial} }}{{\mathrm{n}}_{\mathrm{final}}} \\ \frac{4}{{\mathrm{p}}_{\mathrm{f}}}=\frac{1}{1.5} \end{gathered}$$

Equilibrium pressure of the mixture  = p_f =1.5$\times$4 = 6 atm