$PCl X_{5}$ (calculated vapour density $104.25$ when undissociated) partly dissociates at a total equilibrium pressure of $2.0 atm$ and the vapour density of the mixture is $75$ . Find $K_{p}$ for $PCl X_{5} (g) PCl X_{3} (g) + Cl X_{2} (g)$ in atm, to two decimal places.

Numerical answer type, answer in atm, tolerance 0.01.

Answer: 0.36 atm

α = D / d - 1 = 104.25/75 - 1 = 0.3900

K_{p} = α^{2} P / (1 - α^{2}) = 0.1521 \times 2.0/0.8479 = 0.3588

atm, i.e. 0.36 atm.