[Cryolite]

Consider the cell at 25°C Pt/Ag/AgCl(s)/HCl(0.1m)/Hg2Cl2(s)/Hg/Pt.
I want to find the emf of the cell.

The reactions involved are:

Ag(s) + Cl-    AgCl(s) + e- (oxidation)
2e- + Hg2Cl2(s)    2Hg(l) + 2Cl- (reduction)

The net reaction is 2Ag(s) + Hg2Cl2(s)    2AgCl(s) + 2Hg(l)

Now, according to Nernst, we have E = E° - RT/nF ln[a(AgCl)^2*a(Hg)^2 / a(Ag)^2*a(Hg2Cl2)^2]
We could consider all these activities equal to unity since they are either pure solid or pure liquid, and E = E°, regardless of the concentration of HCl. How could this be? Is there any wrong reasoning here?

[Jack Bauer]

This might make more sense to you if you consider them as half cells i.e., vs. H2/H+ were this is considered to be zero.

Basically Ag(s) + Cl-  right arrow  AgCl(s) + e- (oxidation) will be a Nernest equation where E is a function of Cl- concentration.

http://en.wikipedia.org/wiki/Silver_chloride_electrode