Molar mass Na2SO4 => 142.04 g/mol
Number of moles:
n = sample mass / molar mass
n = 151 / 142.04 => 1.063 moles
Mass of solvent in kilograms :
1 kg = 1000 g
574 g / 1000 = 0.574 kg
Therefore:
b = n solute / m solvent
b = 1.063 / 0.547
b = 1.943 mol/kg
