What is the oxidation number of carbon in methanal?

Calculate the oxidation number of each sulfur atom in the following compounds:
$Na_2S_2O_3$
$Na_2S_4O_6$
$Na_2SO_3$
$Na_2SO_4$

Which one is an incorrect statement?

Ozonolysis of $ClO_2$ produces an oxide of chlorine. The average oxidation state of chlorine in this oxide is . . . . .

What is the oxidation state of nitrogen in $N_3H$?

$KMnO_4$ acts as an oxidising agent in the neutral medium and gets reduced to $MnO_2$. What is the equivalent weight of $KMnO_4$ in a neutral medium?