Equilibrum expressions

Question

PCl₅(g)  PCl₃(g) + Cl₂(g)

a.) write the equillibrium expression.

b and c.) At a certain temperature, a 1.00 L flask initially contained 0.298 mole of PCl3 and 8.70E-3 mole of PCl5. After the system had reached equilibrium, 2.00E-3 mol Cl2 was found in the flask. Gaseous PCl5 decomposes according to the reaction. Calculate the equilibrium concentrations of all species and the value of K

Answer 1

K_c = [PCl₃] [Cl₂] / [PCl₅]

Initial Molarity of PCl₅ = 8.70E-3 mol / 1.00 L = 8.70E-3 M

Initial Molarity of PCl₃ = 0.298 mol / 1.00 L = 0.298 M

PCl₅ decomposes to form 2.00E-3 mol of Cl₂. Therefore, at equilibrium, the concentration of PCl₅ will decrease while the concentrations of PCl₃ and Cl₂ will increase.

[Cl₂]_eq = 2.00E-3 mol / 1.00 L = 2.00E-3 M

[PCl₃]_eq = 0.298 M + 2.00E-3 M = 0.300 M

[PCl₅]_eq = 8.70E-3 M - 2.00E-3 M = 6.70E-3 M

K_c = [PCl₃] [Cl₂] / [PCl₅]

K_c = (0.300) (2.00E-3) / (6.70E-3) = 0.0896