Question

For the decomposition of phosphorous pentachloride at 760 Kelvin, the equilibrium constant is 33.3. You have a sample tube with a volume of 36.3 mL into which you inject 1.50 g of PCl5. What will be the concentration of phosphorous pentachloride, phosphorous trichloride and chlorine at equilibrium?

Answer 1

K = 33.3

PCl5 --> PCl3 + Cl2

K = [PCl3][Cl2]/[PCl5]

initially:

V = 36.3 mL = 0.0363 L;

mol = mass/MW = 1.5/208.2388 = 0.0072 mol

[PCl5] = mol/V = 0.0072 / 0.0363 = 0.1983 M

initially

[PCl5] =0.1983

[Cl2] = 0

[PCl3] = 0

in equilibirum

[PCl5] =0.1983 - x

[Cl2] = 0 + x

[PCl3] = 0 + x

solve for x

K = [PCl3][Cl2]/[PCl5]

33. = x*x/(0.1983 - x)

x^2 = 33.3*0.1983 - 33.3x

x^2 + 33.3x - 6.603 = 0

x = 0.19712

[PCl5] =0.1983 - 0.19712 = 0.00118 M

[Cl2] = 0 + x = 0.19712

[PCl3] = 0 + x = 0.19712