The oxidizing power of [CrO4]2- , [MnO4]2- , and [FeO4]2- follows the order
(1.) [CrO4]2- < [MnO4]2- < [FeO4]2-
(2.) [FeO4]2- < [MnO4]2- < [CrO4]2-
(3.) [MnO4]2- < [FeO4]2- < [CrO4]2-
(4.) [CrO4]2- < [FeO4]2- < [MnO4]2-
Soln (1)
The more an element is electronegative, the higher will be is oxidizing power. More electronegative is Fe (26), followed by Mn (25) and then Cr (24), so the order for the oxidizing power of those elements : Fe>Mn>Cr.
Since oxidation number is same (VI) in all three metals.
[CrO4]2- < [MnO4]2- < [FeO4]2-
Reference https://www.physicsforums.com/threads/a-problem-about-oxdizing-power.827918/