PCI,(g) + Cl2(g) = PCI;(9) K. = 6.5 At a certain point in time, a 1.00 L rigid reaction vessel contains 1.5 mol of PCI.(a). 1.0 mol of CL[g), and 2.5 mol of PCI,(g). Which of the following describes how the measured pressure in the reaction vessel will change and why it will change that way as the reaction system approaches equilibrium at constant temperature? a. The pressure will increase because Q K. C. The pressure will decrease because Q K.