What is the oxidation number of carbon in oxalic acid?

How many peroxide groups are present in $CrO_5$?

Assign the oxidation number to the underlined element in each of the following species:
$1$. $K_2\underline{Mn}O_4$
$2$. $Ca\underline{O}_2$
$3$. $Na\underline{B}H_4$
$4$. $KAl(\underline{S}O_4)_2 \cdot 12H_2O$

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

What is the oxidation state of phosphorus in $Ba(H_2PO_2)_2$?

$2$ 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 $X$?