Phosphorus pentachloride dissociates as follows, in a closed reaction vessel.
PCl5(g) ⇌ PCl3(g) + Cl2(g)
If total pressure at equilibrium of the reaction mixture is P and degree of dissociation of PCl5 is x, the partial pressure of PCl3 will be
$\left(\dfrac{x}{x + 1}\right)P$
$\left(\dfrac{2x}{1 – x}\right)P$
$\left(\dfrac{x}{x – 1}\right)P$
$\left(\dfrac{x}{1 – x}\right)P$