Calculate the amount (in mg) of SeO₃²⁻ in solution on the basis of the following data: 20 ml of M/40 solution of KBrO₃ was added to a definite volume of SeO₃²⁻ solution. The bromine evolved was remove

Question

Calculate the amount (in mg) of SeO₃²⁻ in solution on the basis of the following data: 20 ml of M/40 solution of KBrO₃ was added to a definite volume of SeO₃²⁻ solution. The bromine evolved was removed by boiling and excess of KBrO₃ was back titrated with 7.5 ml of M/25 solution of NaAsO₂. The reactions are SeO₃²⁻ + BrO₃⁻ + H⁺ → SeO₄²⁻ + Br₂ + H₂O; BrO₃⁻ + AsO₂⁻ + H₂O → Br⁻ + AsO₄³⁻ + H⁺ (Se = 79)

TestHub · Accepted Answer

KBrO₃ taken = 20/40 = 0.5 mmol. AsO₂⁻ used = 7.5/25 = 0.3 mmol; since BrO₃⁻ → Br⁻ takes 6e⁻ and AsO₂⁻ → AsO₄³⁻ gives 2e⁻, excess BrO₃⁻ = 0.3/3 = 0.1 mmol. BrO₃⁻ reacting with SeO₃²⁻ = 0.4 mmol; BrO₃⁻ → Br₂ is a 5e⁻ change and SeO₃²⁻ → SeO₄²⁻ is 2e⁻, so SeO₃²⁻ = 0.4×5/2 = 1.0 mmol. Mass = 1.0×127 = 127 mg. Ans 0127.

Chemistry - Redox Reactions Question with Solution | TestHub

Doubts & Discussion