1. The problem statement, all variables and given/known data
From the standard potential Tl^+ + e^- -----> Tl (solid) E^o= -0.336 V
Determine the standard potential of
Tl_2S (solid) + 2e^- -----> 2Tl (solid) + S^2^-
Given that the K_sp for Tl_2S is 1.2x10^-22
***I cannot get latex to put the -22 in the exponent. The Ksp is 1.2 * 10 ^-22
2. Relevant equations
Nernst Equation
E = E^o -(0.05916/n) log(concentration)
3. The attempt at a solution
E = -0.336 -(0.05916/2) log (1.2x10^-22)
E = +0.312
Im wondering if the log (1.2x10^-22) should be log (1/1.2x10^-22)
because the concentration is the products over the reactants. Can someone point me in the right direction? Thanks
Borek
Jul29-08, 05:46 AM
10^{-22} = 10^{-22}
Write Kso formula for Tl2S and solve for [Tl+].
ssb
Jul29-08, 04:45 PM
10^{-22} = 10^{-22}
Write Kso formula for Tl2S and solve for [Tl+].
OK BOREK!!! You have given me some insight. Thank you! Products over reactants. and because the reactants are solid, they will be in the denominator as the number 1. Ok I was able to come up with the following calculation. I would be much appreciated if someone could confirm for me the accuracy. Thanks!
so my answer is -0.98 when using significant figures due to the Ksp being 2 sig figs....right?
Look good?
Borek
Jul29-08, 05:20 PM
No. Do what I told you to do.
ssb
Jul29-08, 05:30 PM
10^{-22} = 10^{-22}
Write Kso formula for Tl2S and solve for [Tl+].
Kso formula
Tl_2S = 2Tl^+ (aq) + S^- (aq)
1.2x10^{-22} = {[Tl^+]^2[S^-]}/[{Tl_2S}]
Tl_2S is a solid so we can remove it from the equation giving me
1.2x10^{-22} = {[Tl^+]^2[S^-]}
So this is where I get confused... is the correct way to do it this:
1.2x10^{-22} = {[x]^2[x]}
x = 4.9 x 10^{-8}
Borek
Jul29-08, 05:37 PM
Solve for Tl+. Not for some x, you are not trying to find out concentration of saturated solution (which you did wrong BTW - concentrations are different, not identical), but concentration of Tl+ as a function of Kso and [S2-].
ssb
Jul29-08, 06:01 PM
Solve for Tl+. Not for some x, you are not trying to find out concentration of saturated solution (which you did wrong BTW - concentrations are different, not identical), but concentration of Tl+ as a function of Kso and [S2-].
Borek thank you so much for your help on this problem and that other problem I posted. Borek im going to level with you... I am very confused. I thought I knew what a Kso was (something to do with solubility) but I don't know much more than that. In post # 3 (my first reply) I did something similar to what a TA did on a similar problem and I was following his steps. Im guessing that the problem is where i am taking the log but I really am not positive.
Can you give me a little bit more of a bump in the right direction please? Thanks.
Borek
Jul29-08, 06:12 PM
Looks like you have problems with simple algebra.
K_{so} = [Tl^+]^2[S^{2-}]
Solving for [Tl+]:
[Tl^+]=\sqrt \frac {K_{so}} {[S^{2-}]}
Now put [Tl+] concentration into the original equation. As the question asks about standard potential, sulfide activity is 1.
ssb
Jul29-08, 06:20 PM
Looks like you have problems with simple algebra.
K_{so} = [Tl^+]^2[S^{2-}]
Solving for [Tl+]:
[Tl^+]=\sqrt \frac {K_{so}} {[S^{2-}]}
Now put [Tl+] concentration into the original equation. As the question asks about standard potential, sulfide activity is 1.
Ok ok ok.....