[doctorisaacs]

So, I've got two questions I need to answer for my post lab. In part A of my lab, my team and I created a concentration cell of lead using the following concentrations:

Well A: 0.010 M Pb(NO3)2, Well B: 0.10 M Pb(NO3)2

We measured the voltage and got an average voltage of 0.139 V. Our hypothesis was that we could use the Nernst equation to estimate what we would get experimentally. The Ecell standard would then be zero for a concentration cell, which in turn would result in the following estimate for Ecell: 0-(0.0592 V/2)*log(0.010M/0.10M)=0.0296 V. This results in a large discrepancy that I am unable to explain. Am I performing my calculations incorrectly?

For part B, we mixed .900 mL of a 0.05M KI solution with .100 mL of a 0.10M Pb(NO3)2 solution in well A, and 1.000 mL of a 0.10 M Pb(NO3)2 solution in well B. The hypothesis was that we could determine the Ksp of PbI2 from the measured voltage potential of the cell using the Nernst equation. Our average potential was 0.225 V from three runs. In all honesty however, I am unclear as to how we can determine K from the Nernst equation in this case with experimentally determined values for Ecell.

So, in summary:
Part A: Our results show a large discrepancy that I am unable to explain. Am I performing my calculations incorrectly?
Part B: How can we determine Ksp from the data provided.

Thank you!

[doctorisaacs]

PS: our temperature was 21.1 degrees celsius.

[doctorisaacs]

I think I may have figured this out with the help of my lab partner. Still however, I am unsure how we got such a high voltage potential for our lead concentration cell in part A. We got a 0.139 V average, when it should have looked more like 0.0296 V. What would explain this 370% error?

Also, for part B, the Ksp calculated was 1.3x10^12 at 294.25 kelvin, where the standard at 293.15 kelvin looks more like 2.14x10^8. What would explain this lower value?

I may have performed my calculations wrong, but if not, why are our values so far off?

[Borek]

0.0296 V for the first part looks OK. 0.139 V is definitely wrong, and suggests either a human error, or device malfunction.

Calculated Ksp looks more or less consistent with the experimental data, but as you got so wrong result in part a, I am not surprised determined Ksp value is so off.

Please don't cross post, I am locking your other thread.

[doctorisaacs]

Sorry about the cross posting, they were related yes but not exactly the same question. Since you locked the other post, let me reiterate what I had said in it:

Human error is a given. I was hoping for something a little more exploratory. A few variables to consider:

1. Used a salt bridge made of natural fiber soaked in KNO3.
2. Data collection for 2 seconds
3. Calibrated with standard Cu and Zn 1M cell, which checked out.
4. Measured room temperature of 21.1 degrees Celsius.

With our original test run with a Zn and Cu cell, we ended up with nearly half the voltage potential than what was ideal. So, we doubled our salt bridge from one fiber to two fibers. Strangely, this ended up resulting in a doubling in cell potential readings, and almost equaling the standard cell potential. So, for the lead concentration cells, we ended up using two salt bridges (two fibers).

With these factors considered, could one of these factors have resulted in the overly high voltage potential? If not one of these factors, what would be the next likely candidates? It should be noted that we ran this experiment multiple times, each time with a fresh run, and each time we ended up with 0.139 V plus or minus .002 V. Human error is extremely broad, so if anyone else has any useful input it would be greatly appreciated.

Now, for part B we ended up with a lower Ksp than what we should have, 1.3x10^-12 versus approx. 2.14x10^8 (text book Ksp at 20 degrees Celsius). Temperature readings of the room were 21.1 degrees Celsius, but I would think this would result in a higher Ksp and not a lower Ksp. We also increased the data collection from 2 seconds to 15 seconds.

Thank you Borek for your assistance. It should be noted that these experiments were conducted multiple times with little variance from run to run. With that, improper calibration or loss of calibration of the LabQuest are absolutely possible sources of error; however, considering that the calibration checked with a Cu Zn cell, other likely causes should be explored. It is also important to note that Part A and B are not related in the calculations that are being done. They are two different experiments, both using lead concentration cells.

If there is anyone else who is able to brainstorm a little more in depth than just "human error", please reply to this post and tell me your thoughts. It would be greatly appreciated!

[Borek]

Temperature doesn't matter - that is, several degrees lower or higher won't change the result by more than single percent. That's not the order of magnitude we are talking about.

With our original test run with a Zn and Cu cell, we ended up with nearly half the voltage potential than what was ideal. So, we doubled our salt bridge from one fiber to two fibers. Strangely, this ended up resulting in a doubling in cell potential readings, and almost equaling the standard cell potential.

Have you tried three salt bridges? What was the salt used?

First thing to check are solutions used (purity, concentration, salt identity) and electrodes.

I am afraid you should be ready for a disappointment. IMHO information you gave so far doesn't give much field for anything, but unsupported speculation. Chances are you have no information for anything else.

If your first lead based concentration cell gave wrong reading, next experiment doesn't make much sense. It is granted it will yield incorrect value of Ksp.

Oh, and the correct Ksp value is in the 10^-8, not 10⁸ range. But that's probably just a typo.

[doctorisaacs]

Yes, 10^-8. Also, the salt bridge was KNO3.