$X + Y \rightarrow$ oleum. The sum of oxidation states of the central atom in $X$ and $Y$ is:

The oxidation state of iodine in $H_4IO_6^-$ is

What is the change in oxidation number of selenium in the following redox reaction?
$SeO_{3(aq)}^{2-} + Cl_{2(g)} + 2OH^- \longrightarrow SeO_{4(aq)}^{2-} + 2Cl_{(aq)}^- + H_2O$

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 . . . . .

The oxidation number of $N$ in $NH_3$ is:

Select the compound in which chlorine is assigned the oxidation number $+5$.