AIIMS 2018 Chemistry Nernst Equation MCQ Question

To find the standard reduction potential $E^\circ$ for the reaction $B^+ + e^- \rightarrow B$, we can use the relationship between the equilibrium constant $K$ and the standard cell potential, given by the Nernst equation. The overall cell reaction can be expressed in terms of $K$ as $E^\circ = \frac{0.059}{n} \log_{10} K$. With $\log_{10} K = 15.6$ and $n = 2$ (since 2 electrons are transferred in the overall reaction), we calculate $E^\circ$ for $B^+ + e^- \rightarrow B$ to be approximately +0.80 V, aligning with option A.

Options B, C, and D are incorrect because they do not match the calculated potential based on the provided equilibrium constant and the stoichiometry of the reactions involved. Specifically, option B (1.26 V) and option D (+0.94 V) suggest higher potentials than what is derived from the equilibrium constant, while option C (−0.54 V) incorrectly indicates a negative potential, which is inconsistent with the positive $K$ value.