I need help on my chemistry homework!! I don't know what I'm doing wrong:/

Question

Phosphorus pentachloride decomposes according to the chemical equation PCl5(g)↽−−⇀PCl3(g)+Cl2(g)𝐾c=1.80 at 250 ∘C A 0.2936 mol sample of PCl5(g) is injected into an empty 4.20 L reaction vessel held at 250 ∘C. Calculate the concentrations of PCl5(g) and PCl3(g) at equilibrium.

Answer 1

PCl₅(g) <==> PCl₃(g) + Cl₂(g) 𝐾c=1.80 at 250ºC

Convert PCl₅ to a molar concentration and then use an ICE table.

[PCl₅] = 0.2936 mol/4.20 L = 0.06990 M

Set up an ICE table:

PCl₅(g) <==> PCl₃(g) + Cl₂(g)

0.06990..........0.............0........Initial

-x...................+x...........+x.......Change

0.0699 -x........x.............x........Equilibrium

Write the equilibrium expression:

Kc = [PCl₃][Cl₂] / [PCl₅] .180

1.80 = (x)(x) / (0.6990 - x)

Solve for x using quadratic equaiton:

0.1258 - 1.80x - x² = 0

x = 0.06737

Equilibrium concentrations:

[PCl₅] = 0.06990 - 0.06737 = 0.002530 M

[PCl₃] = 0.06737 M

[Cl₂] = 0.06737 M