How many grams of Na2SO4 are required to make 350 ml of 0.5M?

Molar mass Na2SO4 = 142 g/mole0.5 M = 0.5 moles/L = 0.5 moles x 142 g/mole = 71 g/L

350 ml = 0.350 L, therefore 71 g/L x 0.35 L = 24.85 g

If using proper significant figures, it would be 20 g