Freezing point depression 2

An aqueous solution is $0.4201$ molal in Na $_{2}$ SO $_{4}$ . It has a freezing point of $-2.109$ °C. Determine the effective number of particles that arise from each Na $_{2}$ SO $_{4}$ .

Use the freezing point depression constant of water $K_f =1.86$ K kg mol $^{-1}.$