What is the volume of 3.5 moles of chlorine gas at standard temperature and pressure (STP)? a) 6.4 L
b) 22 L
c) 32 L
d) 78 L

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.