Question

A battery is constructed of $Cr$ and $Na_2Cr_2O_7$. The unbalanced chemical equation when such a battery discharges is following:
$\ce{Na2Cr2O7 + Cr+H+ -> Cr3+ + H2O + Na+}$
If one Faraday of electricity is passed through the battery during the charging, the number of moles of $Cr^{3+}$ removed from the solution is

• A. $\frac{4}{3}$
• B. $\frac{1}{3}$
• C. $\frac{3}{3}$
• D. $\frac{2}{3}$

Answer 1

Answer: (C)

Reduction half reaction :
$\ce{Cr2O^{2-}_7 + 6e^- + 14H+-> 2Cr^{3+} + 7H2O}$
Oxidation half reaction:
$\ce{Cr->Cr^{3+} +2e^-}$
Overall reaction:
$\ce{Cr2O^{2-}_7 + Cr + 14H+ + 3e^{-}-> 3Cr^{3+} + 7H2O}$
$3F$ of electricity = 3 moles of $Cr^{3+}$
$1\,F$ of electricity $=\frac{3}{3}$ moles of $Cr^{3+}$