Phosphorous pentachloride is a fine grained powder, ranging in colour from colourless to pale yellow. It has a pungent odour. In the gaseous state it shows the equilibrium shown in the equation below. Initially, a sample of phosphorus pentachloride is introduced to an empty vessel. If the equilibrium constant for the above reaction is 4.0 × 10–4 and the equilibrium concentration of [PCl5(g)] is 0.090 mol/L, the equilibrium concentration of [PCl3(g)] is _______ mol/L.

Question

Phosphorous pentachloride is a fine grained powder, ranging in colour from colourless to pale yellow. It has a pungent odour. In the gaseous state it shows the equilibrium shown in the equation below. Initially, a sample of phosphorus pentachloride is introduced to an empty vessel. If the equilibrium constant for the above reaction is 4.0 × 10​–4​ and the equilibrium concentration of [PCl​5​(g)] is 0.090 mol/L, the equilibrium concentration of [PCl​3​(g)] is _______ mol/L.

SHOW ALL YOUR WORK.

DrBob222 · Answer

You didn't show the equation but I believe I have written it below.
...................PCl5(g) ==> PCl3(g) + Cl2(g)
I....................y...................0..............0
C..................-x...................x..............x
E.................y-x...................x..............x

Kc = 4E-4 = (PCl3)(Cl2)/(PCl5)
So y-x = 0.090 M. Substitute 0.090, x, and x into the Kc expression above and solve for x =(PCl3) = (Cl2)