A chemist places 4.00 g of sodium carbonate in a 250.0 ml volumetric flask and fills it up to the mark with water. Calculate the concentration of Na in units of mol/l (m).

A) 0.032 mol/l (m)
B) 0.040 mol/l (m)
C) 0.020 mol/l (m)
D) 0.016 mol/l (m)