In which compound is the oxidation state of nitrogen $-1$?

What are the oxidation states of the most electronegative element in the products of the reaction between $BaO_2$ and $H_2SO_4$?

The oxidation state of sulphur in $SO_4^{2-}$ is

In which of the following compounds,does nitrogen exhibit the highest oxidation state?

The oxidation number of $H$ in $KH$,$MgH_2$,and $NaOH$ are respectively

Write the Lewis structure of the $O_2^-$ ion and find out the oxidation state of each oxygen atom. What is the average oxidation state of oxygen in this ion?