PClX3(g)+ClX2(g)PClX5(g)Kc=6.5\ce{PCl3(g) + Cl2(g) <--> PCl5(g) } \hskip{2em} K_c=6.5

At a certain point in time, a 1.00 L rigid reaction vessel contains 1.5 mol of PClX3(g)\ce{PCl3(g)}, 1.0 mol of ClX2(g)\ce{Cl2(g)}, and 2.5 mol of PClX5(g)\ce{PCl5(g)}. Which of the following describes how the measured pressure in the reaction vessel in the reaction vessel will change and why it will change that way as the reaction system approaches equilibrium at constant temperature?