homework question

Question

Phosphorus pentachloride decomposes according to the chemical equation

PCl5(g)↽−−⇀PCl3(g)+Cl2(g) 𝐾c=1.80 at 250 ∘C

A 0.4186 mol sample of PCl5 (g) is injected into an empty 4.25Lreaction vessel held at 250 ∘C. Calculate the concentrations of PCl5(g) and PCl3 (g) at equilibrium. 

Answer 1

[PCl5] = 0.4186 mol / 4.25 L = 0.09849 M

PCl5(g) <==> PCl3(g) + Cl2(g)

0.09849...........0...............0..........Initial

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

0.09849-x.........x................x.........Equilibrium

Kc = [PCl3][Cl2] / [PCl5]

1.80 = (x)(x) / (0.09849 - x)

x² = 0.1773 - 1.8x

x² + 1.8x -0.1773 = 0

x = 0.09363 M

Equilibrium concentrations:

[PCl5] = 0.09849 - 0.09363 = 0.004860 M

[PCl3] = 0.09363 M