What is the oxidation number of carbon in oxalic acid?

How many peroxide groups are present in $CrO_5$?

In which of the following compounds is the oxidation number of iodine fractional?

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

What is the correct order of oxidation states of sulfur in the following anions: $SO_3^{2-}$,$S_2O_4^{2-}$,and $S_2O_6^{2-}$?

Calculate the oxidation number of sulphur,chromium,and nitrogen in $H_2SO_5$,$Cr_2O_7^{2-}$,and $NO_3^-$. Suggest the structures of these compounds and explain the fallacy in the calculated oxidation number for $H_2SO_5$.