3. 0.30 moles of Br2 (g) and 0.30 moles of Cl2(e) are introduced into a 1.0 L container at 400 K. The equilibrium constant for the reaction
Br2(g) +Ch() +2BrCle)
is Ko= 7.0 at this temperature. Calculate the equilibrium concentrations for
each of the species in this system.

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.