What is the oxidation state of $As$ in $H_{3}AsO_{4}$?

$2$ mole of $N_2H_4$ loses $16$ mole of electrons and is converted to a new compound $X$. Assuming that all of the $N$ atoms appear in the new compound,what is the oxidation state of $N$ in $X$?

The oxidation state of $Fe$ in $Fe_3O_4$ is:

Which of the following statements is not correct?

What is oxidation number?

In which of the following compounds is chlorine in the $+1$ oxidation state?