How many moles of CO2 can be produced by the reaction of 5.0 mol of C2H4 and 12.0 mol of O2? Hint: find Limited reactant 1st
5
(C2H4 + 3 02
2 CO2 + 2 H20)

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.