PLEASE HELP FAST! WORTH 13 POINTS! A student was titrating a 18.00 mL sample of HCl with Ca(OH)2. To reach the endpoint of the titration and neutralize all of the HCl, the student added 49.62 mL of 0.150 M Ca(OH)2. What was the concentration of hydrochloric acid?

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.