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

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.