Moles of KIO3 = 0.2164/214.1 = 0.00101 (3 s.f.)
Moles of Na2S203 = 0.00101 x 6 = 0.00606 (3 s.f.)
Concentration of Na2S203 = 0.00606/6.20 = 0.000977 (3 s.f.) (I use mol/dm-3 instead of mol/L so i don't think it's converted to mol/L from mL) hope this helps
