Ion electron balancing problem

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Homework Statement

The problem is given like this "A student unsuccessfully tried to balance the following equation:
Cr₂O₇^2-+Fe³⁺+H⁺→Cr ³⁺ +Fe ²⁺ + H₂O

Homework Equations

The Attempt at a Solution

First tried to find the oxidation no.
-8 -2 3+ 1+
Cr₂O₇^2-+Fe³⁺+H⁺

 

[Borek]

First tried to find the oxidation no.
-8 -2 3+ 1+
Cr₂O₇^2-+Fe³⁺+H⁺

Show how you got -8.

 

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Thanks for pointing out my mistake !