Balancing FeO(aq) + V2O5(aq) → Fe2O3(aq) + VO(aq) in acidic solution, how many electrons are transferred?

• A. 4
• B. 6
• C. 8
• D. 2

Answer: (B)

You count electrons by tracking how oxidation states change for the elements that actually undergo oxidation and reduction, then tie those changes to the balanced stoichiometry in the acidic medium.

In FeO, the iron is in the +2 state. In Fe2O3, iron is +3. So iron is oxidized, losing one electron per iron atom that ends up in Fe2O3. If two iron centers participate (as implied by the balanced balance with Fe2O3), that accounts for two electrons lost.

Vanadium starts as +5 in V2O5. In VO, vanadium is in the +4 state, so each vanadium atom is reduced by one electron. Each V2O5 molecule has two vanadium atoms; if two V2O5 units participate, that’s four vanadium atoms reduced, consuming four electrons.

Total electrons moved in the redox process = 2 electrons lost by iron + 4 electrons gained by vanadium = 6 electrons.

Thus six electrons are transferred.