Potassium manganate $(K_2MnO_4)$ is not formed in which of the following reactions?

Amphoteric oxide$(s)$ of $Mn$ is/are:

What is the use of potassium dichromate $(K_2Cr_2O_7)$?

The transition metal ions responsible for colour in ruby and emerald are,respectively:

Determine the correct $(T)$ and incorrect $(F)$ statements from the following:
$(a)$ In the titration of $C_2O_4^{2-}$ with $KMnO_4$,$C_2O_4^{2-}$ acts as an oxidizing agent.
$(b)$ $KMnO_4$ is a strong oxidizing agent.
$(c)$ In $Cr_2O_4^{2-}$,the $O-Cr-O$ bond angle is $120^o$.
$(d)$ The standard electrode potential for the reaction $Cr_2O_7^{2-} + 14H^+ + 6e^- \to 2Cr^{3+} + 7H_2O$ is $E^o = -1.33 \ V$.

In ruby and emerald,the metal ion$(s)$ present is/are