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

Which of the following compositions is found in dental amalgam alloys?

Consider the following reactions:
$NaCl + K_{2}Cr_{2}O_{7} + H_{2}SO_{4}$ (conc.) $\rightarrow (A) +$ side products
$(A) + NaOH \rightarrow (B) +$ side product
$(B) + H_{2}SO_{4}$ (dilute) $+ H_{2}O_{2} \rightarrow (C) +$ side product
The sum of the total number of atoms in one molecule each of $(A)$,$(B)$,and $(C)$ is:

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

$X$ is an oxoanion of the lightest element of group $7$ in the periodic table. The metal is in $+6$ oxidation state in $X$. The color of the potassium salt of $X$ is

Highest fluoride and highest oxide of $Mn$ are respectively