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An impure sample of sodium oxalate ( Na 2 C 2 O 4 )   weighing 0 . 20   g is dissolved in an aqueous solution of H 2 SO 4 and the solution is titrated at 70 °   C , requiring 45   ml 0 . 02   M   KMnO 4 solution. The endpoint is overrun, and the back titration is carried out with 10   ml of 0 . 1   M oxalic acid solution. Find the per cent purity of Na 2 C 2 O 4 in the sam

Options

  1. A75
  2. B83 . 75
  3. C90 . 25
  4. DNone of these

Correct answer

B. 83 . 75

Step-by-step solution

The number of milli equivalent of Na 2 C 2 O 4   = milli equivalent of KMnO 4 reacted Total milli equivalent of KMnO 4   - excess milli equivalent of KmnO 4   reacts with H 2 C 2 O 4 = 4 5 × 0.02 × 5 - 1 0 × 0.1 × 2 = 2.5 1000× W 134 ×2 =2.5 W Na 2 C 2 O 4 = 0 .1675 g Percent   purity   of   Na 2 C 2 O 4 in   sample =   0 . 1675   0 . 2 × 100 = 83 .75

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