Oxidation number of potassium in $K_2O$,$K_2O_2$ and $KO_2$,respectively,is

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

What is the difference in the oxidation number of nitrogen when nitric acid is converted into nitrous oxide?

Which one of the following has the highest oxidation number of iodine?

$LiCoO_2$ crystallizes in a rhombohedral structure. Consider a situation where $50\%$ lithium $(Li)$ is extracted from the lattice. To keep the crystal electrically neutral, the change in the average oxidation state of $Co$ is

What is the oxidation number of $Cl$ in $HClO_4$ and $HClO_3$?