The dissociatiion of HI is independent of pressure, while the dissociation of `PCl_(3)` depends upon the pressure applied explain.

Explanation

According to Le Chattler's principle increase in pressure shifts the equilibrium in the direction where the number of the gaseous molecules are less and decrease in pressure shift the equilibrium in the direction where the number of gaseous molecules are more.
`H_(2)(g)+I_(2)(g)hArr2HI(g)`
In this reactioin the sum of the number of gaseous reactant molecules and the total number of gaseous product molecules is equal. So pressure has no effect on this equilibrium.
Dissociation of `PCl_(5)`:
`Pcl_(5)(g)hArrPCl_(3)(g)+Cl_(2)(g)`
In this reaction the no. of gaseous reactant molecules (1) is less than the sum of the gaseous product molecules (2). So increase in pressure shifts the equilibrium in the backward direction. So increase in pressure decreases the dissociation of `Pl_(5)`