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

The sodium salt of an organic acid $'X'$ produces effervescence with conc. $H_2SO_4$. $'X'$ reacts with an acidified aqueous $CaCl_2$ solution to give a white precipitate which decolourises an acidic solution of $KMnO_4$. $'X'$ is:

In the reaction $SnCl_2(excess) + HgCl_2 \rightarrow A + SnCl_4$,'$A$' is

Which one of the following reactions is not feasible?

Among $C$,$S$,and $P$,the element$(s)$ that produce$(s)$ $SO_{2}$ on reaction with hot conc. $H_{2}SO_{4}$ is / are

$SO_2$ can reduce: