# The equilibrium constant (Kp) for the interconversion of PCl5 and PCl3 is 0.0121:

PCl5 (g) ↔ PCl3 (g) + Cl2 (g)

A vessel is charged with PCl5, giving an initial pressure of 0.123 atm. At equilibrium, the partial pressure of PCl3 is __________ atm.

I have a hit a wall with this question, this is where I got to:

PCl5 (g) ↔ PCl3 (g) + Cl2 (g)

I: 0.123 0 0

C: -2x. x x

E: 0.123-2x x x

Kp= [PCl3][Cl2]/[PCl5]

Kp=[x][x]/[0.123-2x]

0.0121=x^2/0.123

and thats where I have gotten stuck can't seem to get the correct answer, I know is 0.0330 but I can not figure out how to exactly get to it. Thanks for any help.