AMU Medical AMU Solved Paper-2012

  • question_answer
    Consider the cell,\[Pt|{{H}_{2}}(g,\,1\,atm)\,{{H}^{+}}(aq,\,\,1M)||F{{e}^{3+}}(aq),\]\[F{{e}^{2+}}(aq)|Pt(s)\] Given that \[{{E}^{o}}_{F{{e}^{3+}}/F{{e}^{2+}}}=0.771\,\,V\], the ratio of concentration of \[F{{e}^{2+}}(aq)\] to \[F{{e}^{3+}}(aq)\] is, when the cell potential is 0.830 V.

    A)  0.101                                   

    B)  0.924

    C)  0.120                                   

    D)  None of these

    Correct Answer: A

    Solution :

                    The half-cell reactions of the given cell are as At anode                 \[\frac{1}{2}{{H}_{2}}\xrightarrow{{}}{{H}^{+}}+{{e}^{-}};\,E_{1}^{o}=-0.00\,\,V\] At cathode                 \[F{{e}^{3+}}+{{e}^{-}}\xrightarrow{{}}F{{e}^{2+}};\,E_{2}^{o}=0.771\,\,V\] ____________________________________ Net reaction                 \[F{{e}^{3+}}+\frac{1}{2}{{H}_{2}}\xrightarrow{{}}F{{e}^{2+}}+{{H}^{+}};E_{cell}^{o}\]                                 \[=0.771\,\,V-0.00\,V=0.771\,\,V\] From Nernst equation,     \[{{E}_{cell}}=E_{cell}^{o}=-\frac{0.0591}{n}\log \frac{[F{{e}^{2+}}]\,[{{H}^{+}}]}{[F{{e}^{3+}}]+pH_{2}^{1/2}}\]                                 \[0.830=0.771-\frac{0.0591}{1}\log \left[ \frac{F{{e}^{2+}}}{F{{e}^{3+}}} \right]\]                                 \[-\frac{0.059}{0.0591}=\log \frac{[F{{e}^{2+]}}}{[F{{e}^{3+}}]}\]                 \[\therefore \]  \[\frac{[F{{e}^{2+]}}}{[F{{e}^{3+}}]}=\]antilog \[(-0.998)=0.101\]


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