Phosphorus pentachloride decomposes according to the chemica

Phosphorus pentachloride decomposes according to the chemical equation PCI5(g) PCI3(g) +CI2(g) Kc = 1.80 at 250 degree C A 0.219 mol sample of PCI5(g) is injected into an empty 2.85 L reaction vessel held at 250 degree C. Calculate the concentrations of PCl5(g) and PCI3) at equilibrium.

Solution

0.219moles / 2.85L = 0.0768M PCl5

Kc = [PCl3][Cl2] / [PCl5]
1.8 = x^2 / 0.0768 - x
x^2 + 1.8x - 0.1382 = 0
quadratic equation
x = [PCl3] = [PCl5] = 0.0737M