Oxidation number of $S$ in $Na_2SO_4$ is

$A$ compound contains atoms of three elements $A$,$B$ and $C$. If the oxidation number of $A$ is $+2$,$B$ is $+5$ and that of $C$ is $-2$,the possible formula of the compound is:

The oxidation state of $Mn$ in $K_2MnO_4$ is:

The oxidation number of $C$ in $CO_2$ is

The equivalent weight of $MnSO_4$ is half of its molecular weight when it is converted into .....

Consider the following molecules: $Br_3O_8$,$F_2O$,$H_2S_4O_6$,$H_2S_5O_6$,and $C_3O_2$. Count the number of atoms existing in their zero oxidation state in each molecule. Their sum is . . . . .