Identify the catalyst used in the reaction between Iodide and persulphate ions.

$$2 \mathrm{I}^{-}+\mathrm{S}_2 \mathrm{O}_8{ }^{2-} \rightarrow \mathrm{I}_2+2 \mathrm{SO}_4{ }^{2-}$$

KMnO$$_4$$ acts as an oxidising agent in acidic medium. The number of moles of KMnO$$_4$$ that will be required to react with one mole of oxalate ions (to form CO$$_2$$) in acidic solution is

Which of the following gas readily de-colourises the acidified KMnO$$_4$$ solution?

The oxidation number of Cr in CrO$$_5$$ which has the following structure, is