First slide

Standard EMF of a cell

Question

A cell constructed by coupling a standard copper electrode and a standard magnesium electrode has emf of 2.7 volts. If the standard reduction potential of copper electrode is + 0.34 volt, that of magnesium electrode is

Moderate

A

+ 3.04 volts
B

– 3.04 volts
C

+ 2.36 volts
D

– 2.36 volts

Solution

Since EMF of cell is +ve. SRP of Anode must be less than SRP of cathode.
SRP of Mg electrode is less than that of Cu electrode.
$E_{cell} = E_{c}^{o} - E_{a}^{o} = E_{{Cu}^{+ 2} / Cu}^{o} - E_{{Mg}^{+ 2} / Mg}^{o}$
$2.7 = 0.34 - E_{{Mg}^{+ 2} / Mg}^{o}$
$E_{Mg + 2 / Mg}^{o} = - 2.36 V$

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