Answer:
The equilibrium concentration of [PClâ ] = 0.24M
Explanation:
This is the reaction.
PClâ (g) â PClâ(g) +Clâ(g)
Now let's make, the expression for Kc
Remember that concentrations must be in M
Kc = ( [PClâ] . [Clâ] ) / [PClâ ]
4.2x10âťÂ˛ = [0.1] . [0.1] / [PClâ ]
[PClâ ] =  [0.1] . [0.1] / 4.2x10âťÂ˛
[PClâ ] = 0.24M