The dark purple colour of KMnO₄ disappears in the titration with oxalic acid in acidic medium. The overall change in the oxidation number of manganese in the reaction is :

Solution

Chemistry ma’am · 3 steps

The purple goes because MnO₄⁻ becomes nearly colourless Mn²⁺: find Mn's number on both sides.

1. Mn in permanganate. In MnO₄⁻, take O as -2 and let Mn be x. Then x + 4(-2) = -1, so x = +7.
2. Mn after the titration. In acidic medium, permanganate is reduced to Mn²⁺, which is almost colourless. That is why the purple colour disappears. Here Mn is +2.

   2MnO₄⁻ + 5H₂C₂O₄ + 6H⁺ → 2Mn²⁺ + 10CO₂ + 8H₂O

3. So. Mn goes from +7 to +2, a change of 7 - 2 = 5 for each Mn atom. That is option (a).

The idea to keep. Permanganate changes by 5 in acid (Mn²⁺), by 3 in neutral or weakly basic medium (MnO₂), and by 1 in strongly basic medium (MnO₄²⁻). Always check the medium first.

Answer from the NTA final answer key. Solution by PrepWiser.