$H_2S$ acts only as a reducing agent while $SO_2$ can act both as a reducing and oxidizing agent because

In the redox reaction $2MnO_4^- + 5C_2O_4^{2-} + 16H^{+} \rightleftharpoons 2Mn^{2+} + 10CO_2 + 8H_2O$,the ion acting as autocatalyst is

$KMnO_4$ acts as an oxidizing agent in weak alkaline medium. When weak alkaline $KMnO_4$ is treated with $KI$,iodide ion is oxidized to $.........$

Which one of the following reactions is not possible?

Upon the addition of a solution $A$ to a strongly acidified solution of barium nitrate,a white precipitate was obtained which did not dissolve even after large addition of water. Solution $A$ contained

$500$ mL of $0.2$ $M$ $MnO_4^-$ solution in basic medium when mixed with $500$ mL of $1.5$ $M$ $KI$ solution, oxidises iodide ions to liberate molecular iodine. This liberated iodine is then titrated with a standard $x$ $M$ thiosulphate solution in presence of starch. If $300$ mL of thiosulphate was consumed, then the value of $x$ is . . . . . . .