WE are given 6.7 moles of PCl₅ in a 1 liter flask. And we are to find the final concentration of Cl₂. Before we can get to it, we have to find the reaction that produces Cl₂.
PCl₅ → Cl₂ + PCl₃
Since the reaction produces equal moles of Cl₂ and PCl₃, then Cl₂ is also equal to 6.7 moles.
Molarity = number of moles of solute/liter of solution
Molarity of PCl₅ = 6.7/1 = 6.7 molarity of Cl₂