The acid-dissociation constants of HC3H5O3(aq) and CH3NH3+(aq) are given in the table above. Which of the following mixtures is a buffer with a pH of approximately 3?

The density would be 1.622

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.