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Question

The standard potential of the following cells is 0.23 V at 15oC and 0.21 V at 35oC.

Pt|H2(g)|HCl(aq)|AgCl(s)|Ag(s)

The solubility of AgCl in water 25oC.

[Note : The standard reduction potential of the Ag+(aq)/Ag(s) couple is 0.80 V at 25oC]

A
1.215×105 mol litre1
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B
4.415×105 mol litre1
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C
2.345×105 mol litre1
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D
None of these
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Solution

The correct option is A 1.215×105 mol litre1
PtH2(g)|HCl(aq)|AgCls|Ag(s)
(i) 12H2H++eAnode
AgCl+eAg+ClCathode
------------------------------------------------------
12H2+AgClH++Ag+Cl
(ii) ΔGo=nEoG=1×0.23×96500=22195J(at 15oC) ---------- (1)
ΔGo=nEoF=1×0.21×96500=20265J(at 35oC) ----------- (2)
Also,
ΔGo=ΔHoTΔSo

Solving (1) and (2) equation we have
22195=ΔHo288×ΔSo
20265=ΔHo308×ΔSo
- - -
------------------------------------------------------------
ΔSo=96.50J
Again, using (1) equation
22195=ΔHo288×(96.5)
ΔH=49987J
ΔHo=49.987kJ

At 298K or 250C
ΔGo=ΔHoTΔSo=(49987)298×(96.5)
ΔGo=21230J=nE0F=1×E0×96500
E0=0.22V

Consider the following reaction at AgCl(s)/Ag electrode
AgAg++e;EoOP=0.8V
AgCl(s)+eAg+Cl;EoRPCl/AgCl/Ag=0.22V
--------------------------------------
Net : AgClAg++Cl
Ecell=EoOPAg/Ag+0.0591log[Ag+]+EoRPCl/AgCl/Ag+0.0591log1[Cl]
Ecell=0 at equilibrium
EoOPAg/Ag++EoRPCl/AgCl/Ag=0.0591log[Ag+][Cl]=0.0591logKsp
0.8+0.22=0.0591logKsp

logKsp=0.580.059=9.83
KspAgCl=1.477×1010
Solubility of AgCl=(Ksp)=(1.477×1010)=1.215×105 mol litre1

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