
Answer:
Concentration of Clâ‚‚ = 0.0255 M
Explanation:
For the chemical reaction of the decomposition of PCl₅ , with initial concentration of PCl₅ = 0 M , PCl₃ = 0.0571 M and Cl₂ = 0.0571 M , and the value of equilibrium constant (kc) = 0.021
The ICE table can be written as -
             PCl₅(g)  ⇄  PCl₃(g)   +   Cl₂(g)
inital moles    0          0.0571      0.0571
at equilibrium   x         ( 0.0571 -x)   (0.0571 - x)
The equilibrium constant (Kc)for the reaction can be written as the -
Kc = [PCl₃][Cl₂]  / [PCl₅]
Kc = (0.0571 - x)(0.0571 - x) / x
Kc = (0.0571 - x)² / x
0.021  = 0.00326041 - 0.1142x + x²
x² - 0.1352 x +0.00326041 = 0
Solving the quadratic equation ,
x = 0.031415
At equilibrium , the concentration of Clâ‚‚ = (0.0571 -x ) = (0.0571 - 0.031415)
the concentration of Clâ‚‚ = 0.0255 M
Hence, The concentration of Clâ‚‚ at equilibrium = 0.0255M