At some temperature, an equilibrium mixture, in a 1.00-L container, involving the chemical system
PCl5(g) == PCl3(g) + Cl2(g)

is found to contain 2.01×1021 molecules of PCl5, 0.00377 mol of PCl3, and 0.519 g of Cl2.
Calculate the equilibrium constant (Keq expressed in terms of the molar concentrations) at this temperature.