The oxidation number of carbon in $H_2C_2O_4$ is:

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

The oxidation number of carbon in $CH_3Cl$ is:

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

$A$ compound of $Xe$ and $F$ is found to have an atomic ratio $Xe : F$ as $0.4 : 2.4$. Find the oxidation number of $Xe$.

What are the oxidation numbers of the underlined elements in each of the following and how do you rationalise your results?
$(a)$ $K\underline {I_3}$
$(b)$ $H_2\underline {S_4}O_6$
$(c)$ $\underline {Fe_3}O_4$
$(d)$ $\underline {C}H_3\underline {C}H_2OH$
$(e)$ $\underline {C}H_3\underline {C}OOH$