The oxidation number of $As$ in $H_2AsO_4^-$ is:

What is the oxidation number of sulfur in $Na_2S_4O_6$?

Which of the following sequences is correct with reference to the oxidation number of iodine?

The compound $YBa_2Cu_3O_7$,which shows superconductivity,has copper in an oxidation state of ........ Assume that the rare earth element Yttrium is in its usual $+3$ oxidation state.

Oxidation number of oxygen in ozone $(O_3)$ is

Fill in the blanks:
$(1)$ During the reaction $CN^{-} \to CNO^{-}$,......... is added.
$(2)$ The maximum positive oxidation number for any chalcogen compound may be ......... .