For the equilbirium reaction:

PCl5 = PCl3 + Cl2

Say initially there is 1.00 mol of PCl5, why would the equilibrium concentration of PCl5=0.80mol/L, of PCl3=0.20mol/L, of Cl2=0.20mol/L? In a graph, how do you know when does PCl5 stop decreasing and reach equilibrium? Can this be calculated? Besides, why would the equilibrium concentraion of PCl5 equal to that of Cl2(ie 0.20mol/L)?

If I start with PCl3 and Cl2 OR start with 3.00 mol of PCl5, would the equilibrium concentration of each entities be the same as the one above? (ie PCl5=0.80mol/L, of PCl3=0.20mol/L, of Cl2=0.20mol/L)

Thank you!