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A solution containing SeO3²− was treated with 70 mL of M/60 KBrO3. The bromine produced was expelled by boiling, and the unreacted KBrO3 was then back-titrated with 12.5 mL of M/25 NaAsO2. Using the reactions below, determine the amount of SeO3²− present in millimoles. (1) SeO3²− + BrO3− + H+ → SeO4²− + Br2 + H2O (2) BrO3− + AsO2− + H2O → Br− + AsO4³− + H+

1. 1.6 × 10⁻³
2. 1.25
3. 2.5
4. None of these

Correct answer: 2.5

Solution

The correct option is 2.5 millimoles because the amount of KBrO3 used in the reaction can be calculated, and the stoichiometry of the reactions indicates that each mole of SeO3²− reacts with one mole of BrO3−. By determining the moles of unreacted KBrO3 through back-titration with NaAsO2, we can find that 2.5 millimoles of SeO3²− were initially present.

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