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A galvanic cell which makes use of the following reaction at 298K. $Cr_2O_7^{2-}+14H^++6Fe^{2+}\rightarrow 2Cr^{3+}+6Fe^{3+}+7H_2O$.Given : $E^{\large\circ}(Cr_2O_7^{2-},H^+,Cr^{3+}|Pt)=1.33V;E^{\large\circ}(Fe^{3+},Fe^{2+}|Pt)=0.77V$.Increasing $[Fe^{3+}]$ to 2.0M keeping all other concentrations at unity,the emf of the cell is changed by

$\begin{array}{1 1}-0.059V\\-0.0178V\\0.059V\\0.0178V\end{array} $

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Answer : $-0.0178V$
We have
$E=E^{\large\circ}-\large\frac{RT}{6F}$$\ln (2)^6=E^{\large\circ}-\big(\large\frac{6\times 0.059V}{6}\big)$$(-0.301)$
$\Rightarrow E^{\large\circ}-0.0178V$
answered Jul 21, 2014 by sreemathi.v

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