The strength of an I2 solution may be verified by reacting the I2 with thiosulfate.

I2 + 2(SO2O3) == 2I + S4O6
(I is for iodine)
15.0 mL of an iodine solution of unknown molarity was titrated against a 0.00100 M solution of S2O3. The blue black iodine starch complex just disappeared when 20.0 mL of thiosulfate was added. What is the exact molarity of the I2 solution?