The concentration of potassium ions inside a biological cell is at lea...
Solution:
Given, the concentration cell is M(s) | M(aq, 0.05 M) || M(ag, 1 M) | M(s)
The potential difference across the cell is |Ecell| = 70 mV
To find: The new potential difference when 0.05 M M(aq) is replaced with 0.0025 M M(aq)
We can use the Nernst equation to calculate the potential difference of the cell in the new condition:
Ecell = E°cell - (RT/nF) ln Q
where E°cell is the standard potential difference, R is the gas constant, T is the temperature, n is the number of electrons transferred in the cell reaction, F is the Faraday constant, and Q is the reaction quotient.
In this case, the cell reaction is:
M(s) + M+(aq, x M) → M+(aq, 0.05 M) + M(s)
The reaction quotient is:
Q = [M+(aq, 0.05 M)] / [M+(aq, x M)]
Substituting the values in the equation:
Ecell = E°cell - (RT/nF) ln [M+(aq, 0.05 M)] / [M+(aq, x M)]
At 25°C, R = 8.314 J/mol K, n = 1 (since only one electron is transferred), and F = 96,485 C/mol.
E°cell can be calculated from the standard reduction potentials of M+:
E°cell = E°red (M+(aq, 0.05 M)) - E°red (M(s))
Assuming that M+ is a cation, we can use the reduction potential of M2+/M:
E°red (M2+/M) = -2.37 V
E°red (M+(aq, 0.05 M)) = E°red (M2+/M) - (RT/nF) ln [M+(aq, 0.05 M)] / [M]
E°red (M+(aq, 0.05 M)) = -2.37 V - (0.0257 V) ln 0.05
E°red (M+(aq, 0.05 M)) = -2.37 V + 0.478 V = -1.89 V
E°cell = -1.89 V - (-2.37 V) = 0.48 V
Substituting the values in the Nernst equation:
Ecell = 0.48 V - (0.0257 V) ln [M+(aq, 0.05 M)] / [M+(aq, 0.0025 M)]
Ecell = 0.48 V - 0.211 V = 0.269 V
Therefore, the magnitude of the cell potential when 0.05 M M(aq) is replaced with 0.0025 M M(aq) is 0.269 V or 269 mV.
Answer: c) 140 mV
To make sure you are not studying endlessly, EduRev has designed JEE study material, with Structured Courses, Videos, & Test Series. Plus get personalized analysis, doubt solving and improvement plans to achieve a great score in JEE.