What mass of sodium sulfate is needed to make 2.5 L of 2.0 M solution?
A 120 g
B 150 g
C 180 g
D 710 9
E 750 g

D) 710 g

Step by Step:
Multiply 2.5 L by 2.0 M to solve for moles
2 mol/L • 2.5 L= 5 mol

Find formula weight of sodium sulfate

Na2SO4-142.04 g/mol

Na- 2(22.99)=45.98
S-32.06
O-4(16)=64

Multiply miles by formula weight

5 mole • 142.04 g/mol=710.2 g

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.