The oxidation number of $S$ in $H_2S_2O_8$ is

In which compound is the oxidation state of phosphorus $+4$?

What is the change in oxidation number of $S$ in the following reaction?
$H_2S + NO_3^- \longrightarrow H_2O + NO + S$

In acidic medium,$H_2O_2$ changes $Cr_2O_7^{2-}$ to $CrO_5$ which has two $(-O-O-)$ bonds. Oxidation state of $Cr$ in $CrO_5$ is

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

$2.5 \ mol$ of hydrazine $(N_2H_4)$ loses $25 \ mol$ of electrons to form a new compound $X$. Assuming all lost electrons are contained in the new compound,what is the oxidation state of nitrogen in compound $X$?