Where is potassium dichromate used?

What is the starting material for the preparation of $KMnO_4$?

The oxoanion in which the oxidation state of the central atom is same as its group number in the periodic table is

The spin-only magnetic moment value of the most basic oxide of vanadium among $V_2O_3$,$V_2O_4$,and $V_2O_5$ is $..... \ B.M.$ (Nearest Integer).

Boiling $CuCl_2$ with $Cu$ in concentrated $HCl$ gives:

An acidified solution of potassium chromate was layered with an equal volume of amyl alcohol. When it was shaken after the addition of $1 \ mL$ of $3 \% \ H_2O_2$,a blue alcohol layer was obtained. The blue color is due to the formation of a chromium$(VI)$ compound '$X$'. What is the number of oxygen atoms bonded to chromium through only single bonds in a molecule of $X$ ?