The correct option is D −27.3
It is a metal-metal insoluble salt electrode.
Here,
Cd2+(aq) + 2e−→Cd(s)....(1); Eo = −0.403 V
CdS(s) + 2e−→Cd(s)+S 2−(aq).....(2); Eo = −1.21 V
Reversing equation (2), we get,
Cd(s) + S2−(aq)→CdS(s) + 2e−......(3); Eo = 1.21 V
Add equation (1) and (3)
Cd2−(aq) + S2−(aq)→CdS(s).....(4); E0 = 0.807
Reverse equation (4)
⇒CdS(s)→Cd2−(aq) + S2−(aq)....(5); Eo = −0.807 V
For equation (5), standard free energy will be (at equilibrium)
ΔG0=−RT ln Ksp
Also,
ΔG0=−nFE0
∴
−RT ln Ksp=−nFE0
RT ln Ksp=nFE0
E0=2.303×RTnF log10 Ksp
E0 = 0.05922×log10Ksp
log10Ksp=−0.807×20.0591≈−27.3