an aqueous solution containing 0.047 M of S2O82- ion and 0.072 M of I- is prepared and the progress of the reaction followed by measuring [S2O82-]. The peroxydisulfate ion (S2O82-) reacts with the iodide ion in aqueous solution according to the following reaction: S2O82-(aq) 3I-(aq)→ 2SO42-(aq) I3-(aq) The following data are obtained for [A] as the reaction proceeds:

Time (s): 0, 12.8, 39.7
[S2O82-] (M): 0.047, 0.042, 0.033

The concentration of I- remaining at 39.7 s is