Which one of these is not known?

Which of the following are basic oxides among $Mn_2O_7 (I)$,$V_2O_3 (II)$,$V_2O_5 (III)$,$CrO (IV)$,and $Cr_2O_3 (V)$?

How would you account for the increasing oxidising power in the series $VO_2^+ < Cr_2O_7^{2-} < MnO_4^-$?

Which transition metal reduces steam to evolve hydrogen?

Iron is rendered passive by the action of

Transition metals vary widely in their chemical reactivity. Explain this with examples or write about the reactivity of $3d$ series metals with acids.