The reagent used for toning in photography is

The total number of $Mn=O$ bonds in $Mn_2O_7$ is

When $Cu^{2+}$ ion is treated with $KI$,a white precipitate,$X$ appears in solution. The solution is titrated with sodium thiosulphate,the compound $Y$ is formed. $X$ and $Y$ respectively are

What is the use of potassium permanganate $(KMnO_4)$?

Thermal decomposition of a $Mn$ compound $(X)$ at $513 \ K$ results in compound $Y$,$MnO_2$ and a gaseous product. $MnO_2$ reacts with $NaCl$ and concentrated $H_2SO_4$ to give a pungent gas $Z$. Identify $X, Y$ and $Z$.

Which one of the following statements is correct when $SO_2$ is passed through acidified $K_2Cr_2O_7$ solution?