Keq for a Gas-Phase Reaction

A 7.00-L vesselcontained 9.08 × 10⁻² mol of gaseous PCl3, 1.02 × 10⁻¹ mol ofgaseous PCl5, and 1.02 × 10⁻¹ mol of Cl2 gas atequilibrium at 223°C.
Calculate the value of Keq (expressed in terms of the molarconcentrations) for the reaction
PCl3(g) + Cl2(g) ⇌ PCl5(g)


Solution :
[PCl3] = (9.08 × 10⁻²)/7.00 = 0.0908/7 mol/L
[PCl5] = [Cl2] = (1.02 x 10⁻¹)/7.00 = 0.102/7 mol/L

Keq = [PCl5]/([PCl3] x [Cl2]) = 1/[PCl3]= 1/(0.0908/7) = 77.1 (mol/L)⁻¹