A1.0 liter container is filled with 0.300 m of
pcl5 at 250◦c. the vessel is then held at a
constant temperature until the reaction
pcl5(g) ⇀↽ pcl3(g) + cl2(g)
comes to equilibrium. it is found that the
vessel contains 0.200 moles of pcl5. what is
the value of the equilibrium constant for the
reaction at this temperature?

If there is 1.0 liters of 0.300 M at the beginning, that means there was 0.300 moles (because 1 L times 0.300 moles/L = 0.300 mol).  It says that, at equilibrium, there is 0.200 mol PCL5.  That means 0.100 mol PCl5 reacted.  Therefore, 0.100 mol each was created of PCl3 and Cl2.

Equilibrium constant Kc can be found with this: