[h2so4]

Cu(NO3)2 + Na2S = CuS + 2Na(NO3)

Ksp of CuS = 6 x 10power of-36 (as given).

M of S- is given as 2.5 x 10power of-3.

What is the molarity of Cu2+ given the above molarity of S- and the solubility constant for CuS?

Anyone have any ideas?

I've been trying to use the equation: [Cu2+] = Ksp/[S2-] and the answer I get is 2.4x10 power of -33. This doesn't look right to me. Eventually I need to use value of the [Cu2+] in Nernst to find Cell potential.

[Borek]

I've been trying to use the equation: [Cu2+] = Ksp/[S2-] and the answer I get is 2.4x10 power of -33.

That's OK - unless you should take S^2- hydrolysis into account, or sulfide was a limiting reagent.

[h2so4]

 

Thank you Borek. I just could not get  my head around that figure. The Nernst equation that I was plugging that into gave Ecell of 0.888V and the measured V (from an experiment) was 0.050V, which is way way too different. I'm happy now with the Ksp calculation for molarity of Cu2+, now I just have to find a reason for the massive difference in E potential between calculated and measured. 

[Borek]

What was pH of the solution?

On this pH of polyprotic acid page equation 9.13 gives you a way of finding real S^2- concentration in the solution as a function of pH. Unless pH was substantially higher then 12 your S^2- concentration was much much lower than analytical concentration of sulfide.

[h2so4]

Unfortunately with the experiment we did not measure pH, I agree that this would have been a good check to make sure that the calculated concentration was accurate. pH values and calculations are so much easier to understand!

In the experiment, the task was to create a cell from ultra dilute Cu²⁺ solution. We started with 4mL of 0.1M KNO₃ to which an initial 0.10mL of 0.1M Cu(NO₃)² was added. To this solution we added approximately .10mL of 0.1M Na₂S (the addition of the S^- was to reduce the Cu₂₊ to a very low level, presumably to do with the fact that CuS is very insoluble). The voltage of the cell was then measured - oh and we used 0.1M KNO₃ for the salt bridge.

So, having this information we were then asked to calculate the [Cu²⁺] remaining in solution by using a given Ksp for CuS (for which texts actually provide as 8x10^-37 and not the 6x10^-36 as given).

I'm a very mature age student and am finding it really tough going getting my head around this stuff. Some things sit quite well, other things like Ksp, reaction mechanisms etc I struggle with. I have been browsing these forums since February (never posted before now ) and have got a wealth of help with various things just by reading the old posts and the various other things available. I really do appreciate your time and energy in helping all of us with this 

[Borek]

First of all - even ignoring pH - your approach was not correct. You have used stoichiometric amounts of copper and sulfide. What should happen in ideal situation?

But the way I see it it is not possible to calculate concentration of copper in your cell. Concentrations of both solutions are known with finite accuracy and 0.1mL is measured with finite accuracy. Amounts of both substances were measured with some error, so there was slight excess of either copper or sulfide. This slight error gives HUGE difference when it comes to final copper ions concentration. Think about copper concentration when there is 1% excess of copper compared to 1% excess of sulfide. Ignoring pH the difference is (if I am not mistaken) about 10²⁷.

[h2so4]

Thanks again for your help Borek 

I'm waiting for the lecturer to return my work - I think I gave up in the end as I had run out of time and went with the "bad" figure. When I get the work back I'll post up the outcome. I would love to get your thoughts on what the lecturer provides as feedback, thankfully (or not) I am not the only one in this course who is SO confused with this stuff.