The two reactions involved in quantitatively determining the amount of iodate in solution are:
IO3-(aq) + 5 I-(aq) + 6 H+(aq) --> 3 I2(aq) + 3 H2O(l)
followed by reaction of the I2: I2(aq) + 2 S2O32- --> 2 I-(aq) + S4O62-(aq).
What is the stoichiometric factor, that is the number of moles of Na2S2O3 reacting with one mole of KIO3?