How many moles of oxygen (o) are in a sample of 3.98x10^23 atoms

1 mol 6,02·10²³ 
X 3,98·10²³
X = 3,98·10²³/6,02·10²³ 
X= 0,6611 moles of oxygen

:)

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.