A 0.72-mol sample of PCl5 is put into a 1.00-L vessel and heated. At equilibrium, the vessel contains 0.40 mol of PCl3 (g) and 0.40 mol of Cl2 (g). Calculate the value of the equilibrium constant for the decomposition of PCl5 to PCl3 and Cl2 at this temperature.

Equilibrium constant is 0.4

Explanation:

We propose the equilibrium:

                 PCl₅(g)    ⇄    PCl₃(g)   +    Cl₂(g)

Initially    0.72 mol

We have the 0.72 moles of the PCl₅.

React           x                      x                  x

X amount has reacted, so in the equilibrium we have:

               0.72 - x              0.4               0.4

If we initially have 0.72 moles and we have 0.4 moles at the equilibrium, it means that 0.3 moles has been reacted.

Let's make the expression for Kc:

Kc = [PCl₃] . [Cl₂] /  [PCl₅]

Kc =  0.4 . 0.4 / 0.4

Kc = 0.4

the answer is 1 i believe