At equimolar concentrations of `Fe^(2+)` and `Fe^(3+)`, what must `[Ag^(+)]` be so that voltage of the galvanic cell made from `Ag^(+)//Ag` and `Fe^(3+)//Fe^(2+)` electrodes equals zero ? The reaction is `Fe^(2+)+Ag^(+) hArr Fe^(3+)+Ag`. Determine the equilibrium constant at `25^(@)C` for the reaction. `("Given: "E_(Ag^(+)//Ag)^(@)=0.799" volt and "E_(Fe^(3+)//Fe^(2+))^(@)=0.771" volt")`

Explanation

`E_(cell)^(@)=E_(Fe^(2+)//Fe^(3+))^(@)+E_(Ag^(+)//Ag)^(@)`
`=-0.771+0.799=0.028` volt
At equilibrium, `E_(cell)=0`
`0=E_(cell)^(@)-0.0591/1 "log" ([Fe^(3+)])/([Fe^(2+)][Ag^(+)])`
`=E_(cell)^(@)-0.0591"log"1/([Ag^(+)])`
`[Ag^(+)]=0.34`
`log K=(nE^(@))/(0.0591)," "K=3.0`