The Iodine "Clock Reaction" Involves the Following Sequence of Reactions

question 17

Multiple Choice

The iodine "clock reaction" involves the following sequence of reactions occurring in a reaction mixture in a single beaker. IO₃(aq) + 5I^-(aq) + 6H⁺(aq) → 3I₂(aq) + 3H₂O(l)
I₂(aq) + 2S₂O₃^2-(aq) → 2I^-(aq) + S₄O₆^2-(aq)
The molecular iodine (I₂) formed in reaction 1 is immediately used up in reaction 2, so that no iodine accumulates. In one experiment, a student made up a reaction mixture which initially contained 0.0020 mol of iodate ions (IO₃⁻) . If the iodate ions reacted completely, how many moles of thiosulfate ions (S₂O₃²⁻) were needed in reaction 2, in order to react completely with the iodine (I₂) produced in reaction 1?

• A)0.0020 mol
• B)0.012 mol
• C)0.0040 mol
• D)0.0010 mol
• E)0.0060 mol

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