Fe3O4 is a mixed oxide(Fe+2O.Fe+32O3)
The net change in oxidation can be calculated as follows,
Fe3O4⟶+8e−3Fe
For 3 moles of formation of iron, 8 moles of electrons are involved.
One mole of electrons charge is called one Faraday.
Charge for 1 mole Fe=38F
=2.67F