The addition of hydrofluoric acid and to water produces a buffer solution. a. hcl b. nano3 c. naf d. nacl e. nabr

The  addition  of  hydrofluoric  acid  and  Naf( answer  c)  to  water  produces  a  buffer  solution.

Naf  is  used  because  buffer  consist  of  a  weak  acid  and  the salt  of  that  base.Buffer  also  consist  a weak   base  and  salt  of  that  base .Hydrofluoric  acid  is   a  weak  acid  thus  the  salt    would   be  added  is  Naf  to  produce  buffer  solution.

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.