The peroxydisulfate ion (S₂O₈^2–) reacts with the iodide ion in aqueous solution via the reaction:

S₂O₈^2–(aq)  +  3 I^–(aq)    2 SO₄(aq)  +  I₃^–(aq)

An aqueous solution containing 0.050 M of S₂O₈^2– ion and 0.072 M of I^– is prepared, and the progress of the reaction followed by measuring [I^–]. The data obtained is given in the following table:

The concentration of S₂O₈^2– remaining at 400 s is ________ M