the freezing point of water is -1.86 degree Cm, if 5.00 g Na2SO4 is desolve at 45.0g H2o the freezing point is changed by -3.82 degree C. Calculate the Van't Hoff Factor for Na2SO4

Answer 1

I think you meant to say the freezing point depression constant is -1.86 degree C/m for water. (The normal freezing point is 0.00 C)
mols Na2SO4 = grams/molar mass = approx 0.035 but you need to do it more accurately.
molality = mol/kg solution = approx 0.035/0.045 = approx 0.8
delta T = i*Kf*m
3.82 = i*1.86*0.8
Solve for i.