We place 6.7 mol pcl5 in a 1.0 l flask and allow it to reach equilibrium at a given temperature. what is the final concentration of cl2 in the flask?

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

☆ionisation energy increases across a period from na -> ar☆ionisation energy decreases down a group from li -> fr☆ionic radius decreases across a period from na -> cl☆ionic radius increases down a groupfrom f -> ii hope it clarifies your doubts