85. Solving a chemistry problem

3.a) How is the equivalent of iodine thiosulfate determined? How should these solutions be prepared and stored? Under the influence of what factors and according to what equations does sodium thiosulfate decompose?
b) A portion of 0.7840 g of sodium sulfite was dissolved in a volumetric flask with a capacity of 250 cm3, 50 cm3 of 0.045 N iodine solution was added to 25 cm3 of this solution. 25.00 cm3 of 0.05 N thiosulphate solution was used for titration of the resulting mixture. What percentage of Na2SO3 did the sample contain?