How do I figure these out?

A solution of sodium thiosulfate, Na2S2O3, is 0.1365 M. 29.28 mL of this solution reacts with 39.79 mL of I2 solution. What is the molarity of the I2 solution?

2(Na2S2O3) + I2↔Na2S4O6 + 2(NaI)


16.77 mL of the I2 solution from above is required to titrate a sample containing As2O3. Calculate the mass of As2O3 (197.8 g/mol) in the sample.

As2O3 + 5(H2O) + 2I2 → 2(H3AsO4) + 4HI