Write the preparation of following :
(i) `KMnO_4` from `K_2MnO_4`
(ii) `Na_2CrO_4` from `FeCr_2O_4`
(iii) `Cr_2O_(7)^(2-)` from `Cr_2O_(4)^(2-)`

Explanation

(i) `K_2MnO_4 overset("Electrolytic oxidation in")underset("Alkaline solution")to KMnO_4`
(ii) `4FeCr_3O_4+8Na_2CO_3+7O_2tounderset("Water soluble")(8Na_2CrO_4)+underset("Water insoluble")(2Fe_2O_3+)8CO_2`
`2CrO_4^(2-) +2H^(+)to Cr_2O_7^(2-)+H_2O`