The oxidation state of chlorine in perchloric acid $(HClO_4)$ is:

One mole of $N_2H_4$ loses $10 \ moles$ of electrons to form a new compound $Z$. Assuming that all the nitrogen atoms appear in the new compound,what is the oxidation state of nitrogen in $Z$? (There is no change in the oxidation state of hydrogen.)

What are the oxidation states of the two sulfur atoms in thiosulfuric acid $(H_2S_2O_3)$?

In which pair of species is the oxidation number of chlorine the same?

What are the oxidation numbers of nitrogen in $NH_4NO_3$?

What is the oxidation state of oxygen in $O_2^{2-}$ ion?