What is the oxidation number of oxygen in $OF_2$?

In acidic medium,$H_2O_2$ changes $Cr_2O_7^{2-}$ to $CrO_5$ which has two peroxy linkages. The oxidation state of $Cr$ in $CrO_5$ is

The oxidation number of sulphur in $H_2SO_4$ is

What is the oxidation number of sulfur in $H_2S_2O_8$?

$A$ compound contains atoms of $x, y$ and $z$ elements. Oxidation numbers of $x, y$ and $z$ are $+3, +5$ and $-2$ respectively. Identify the possible formula of this compound?

$3 \ mole$ of $N_2H_4$ loses $30 \ mole$ of electrons to form a new compound. Calculate the oxidation state of $N$ in the new compound,assuming the oxidation state of $H$ remains unchanged.