Arsenic(III) oxide (As 2 O 3 ) is available in pure form and is a useful (but carcinogenic) primary standard for oxidizing agents such as MnO 4 – . The As 2 O 3 is dissolved in base and then titrated with MnO 4 – in acidic solution. A small amount of iodide (I – ) or iodate (IO 3 – ) is used to catalyze the reaction between H 3 AsO 3 and MnO 4 – .
As 2 O 3 + 4 OH – ⇌ 2 HAsO 3 2- + H 2 O
HAsO 3 2- + 2 H + ⇌ H 3 AsO 3
5 H 3 AsO 3 + 2 MnO 4 – + 6 H + → 5 H 3 AsO 4 + 2 Mn 2+ + 3 H 2 O
(a) A 3.209 g aliquot of KMnO 4 (FM 158.034) was dissolved in 1.000 L of water, heated to cause any reactions with impurities to occur, cooled and filtered. What is the theoretical molarity of this solution if no MnO 4 – was consumed by impurities?
(b) What mass of As 2 O 3 (FM 197.84) would be just sufficient to react with 25.00 mL of the KMnO 4 solution in part (a)?
(c) It was found that 0.146 8 g of As 2 O 3 required 29.98 mL of KMnO 4 solution for the faint color of unreacted MnO 4 – to appear. In a blank titration, 0.03 mL of MnO 4 – was required to produce enough color to be seen. What is the molarity of the permanganate solution?
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