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A battery is constructed of Cr and Na2Cr2O7. The unbalanced chemical equation when such a battery discharges is following: Na2Cr2O7 + Cr + H+ → Cr3+ + H2O + Na+ If one Faraday of electricity is passed through the battery during the charging, the number of moles of Cr3+ removed from the solution is

1. 4/3
2. 1/2
3. 3
4. 3/4

Correct answer: 3/4

Solution

The balanced equation shows that for every 3 moles of Cr3+ produced, 1 mole of Cr is oxidized, meaning that passing one Faraday (which corresponds to 1 mole of electrons) results in the removal of 3/4 moles of Cr3+ from the solution.

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