Which of the following elements is doped into a fiber amplifier in an optical fiber communication system?

Describe the oxidising action of potassium dichromate and write the ionic equations for its reaction with:
$(i)$ iodide
$(ii)$ iron $(II)$ solution and
$(iii)$ $H_{2}S$

The oxidation states of transition metal atoms in $K_{2}Cr_{2}O_{7}$,$KMnO_{4}$ and $K_{2}FeO_{4}$,respectively,are $x$,$y$ and $z$. The sum of $x$,$y$ and $z$ is

Identify the correctly matched pairs:
$(i)$ $TiO_2$ - pigment industry
$(ii)$ $MnO_2$ - dry battery cells
$(iii)$ $Cu/Ni$ alloy - $UK$ 'copper' coins

$A \xrightarrow{4 KOH, O_2} 2 B (Green) + 2 H_2O$
$3 B \xrightarrow{4 HCl} 2 C (Purple) + MnO_2 + 2 H_2O$
$3 C \xrightarrow{H_2O, KI} 2 A + 2 KOH + D$
In the above sequence of reactions,$A$ and $D$,respectively,are:

Which product is formed by the reaction of an acidic solution of chromic acid with hydrogen peroxide?