124. Solving a chemistry problem

6. a) How is the equivalent of thiosulfate determined? Why is it necessary to prepare a solution of thiosulfate in boiled water and store it in a dark bottle? How does the decomposition of thiosulfate proceed under the action of carbon dioxide?
b) What sample of K2Cr2O7 should be taken to establish a titer of 0.1 N. Na2S2O3 solution, if you have a volumetric flask with a capacity of 200 cm3 and a pipette with a capacity of 10 cm3 and strive to ensure that 25 cm3 of a thiosulfate solution is consumed during the titration of the released iodine?