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To find the standard potential of M3+/M electrode, the following cell is constituted:Pt/M/M+(0.001molL1)/Ag+(0.01molL1)/Ag . The emf of the cell is found to be 0.421volt at 298K . The standard potential of half reaction M+3+3eM at 298K will be:
(Given:EeAg+/Ag at 298K = 0.80Volt)
A.0.32 Volt
B.0.66 Volt
C.1.28 Volt
D.0.38 Volt

Answer
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Hint: The device used to generate electricity from chemical reactions or vice versa is known as an electrochemical cell. The standard emf of a cell is the difference of the standard potentials at the cathode and the anode.

Complete step by step answer:
An electrochemical cell is a device used to generate electricity from chemical reactions or use electricity to proceed the chemical reactions.
As we all know that electricity and the potential difference have relation in them similarly here we will see the term cell potential, basically it is the measure of potential difference between the two half cells of an electrochemical cell. It is also represented as Ecell . This potential difference is developed when there is flow of electrons from one half cell to another.
When this potential difference is measured under standard conditions, it is termed as standard potential. The standard conditions are :the concentration should be 1M , pressure should be 1atm and the temperature should be 25C .
In this problem we have to calculate the standard potential of half reaction. The formula to calculate standard potential is:
Ecell=Ecell0.6nlog[M+3][Ag+]3
Where,
Ecellis the emf of a cell.
Ecell is the standard emf of a cell.
n is the number of electrons involved in reaction.

And we have given that Ecell=0.421volt,
[M+3]=0.001 and [Ag+]=0.01. The number of electrons involved in reaction are three so n=3 .
So by putting all the values in formula we get
0.421=Ecell0.63log[0.001][0.01]3
By solving we get.
Ecell=0.48V
Now,
We all know that the standard emf of a cell is the difference of the standard potentials at the cathode and the anode.
Therefore, Ecell=EAgEM
We have obtained the value of Ecell which is 0.48volt.
And we have given that EAg=0.80V
So by putting all the values we get,
0.48=0.80EM
EM=0.800.48=0.32V
Hence the standard potential of the half reaction M+3+3eM is 0.32V.
So, option (A) is correct.

Note:
The conductivity or conductance of a cell is measured experimentally from the electrical resistance produced by the electrolyte. An alternating voltage is applied across the two electrodes present in electrolyte and ions are allowed to move toward the electrodes and the resistance is noted.