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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: chanelo on March 09, 2013, 04:36:35 PM

Title: Calculating Concentration from Cell Potential
Post by: chanelo on March 09, 2013, 04:36:35 PM

Thanks for reading!
Ok so using the Nernst equation, I'm trying to find the value of [Fe²⁺] in the equation Fe²⁺(aq) + Zn(s)  :rarrow: Fe(s) + Zn²⁺(aq), where [Zn²⁺] is 0.5M.
The experimental value of Ecell was 0.52 V and the value of E°cell is 0.32 V.

Here's what I've done:
0.52 V = 0.32 V - (2.303RT/nF) logQ
0.52 V = 0.32 V - (0.0592 V/2) logQ
0.20 V = -0.0296 log [1][0.5]/[1][Fe²⁺]
0.20 V/0.0296 = log [0.5]/[Fe²⁺]
6.8 = log [0.5]/[Fe²⁺]
10^6.8 = [0.5]/[Fe²⁺]
[Fe²⁺] = [0.5]/6309573
[Fe²⁺] = 0.8 x 10^-7

BUT when I enter this value into the equation 0.32 V - (2.303RT/nF) log [0.5]/[0.8 x 10^-7] I get a value of 0.11, which isn't what I should be getting for Ecell (0.52 V).
Can someone tell me where I went wrong?

Title: Re: Calculating Concentration from Cell Potential
Post by: Borek on March 09, 2013, 05:23:03 PM

Quote from: chanelo on March 09, 2013, 04:36:35 PM

0.20 V = -0.0296 log [1][0.5]/[1][Fe²⁺]
0.20 V/0.0296 = log [0.5]/[Fe²⁺]

What happened to the sign?

I am not saying everything else is correct...

Please format your equations using LaTeX, as signaled here: http://www.chemicalforums.com/index.php?topic=59314.0

Title: Re: Calculating Concentration from Cell Potential
Post by: chanelo on March 10, 2013, 02:31:03 PM

Thank you!! I kept the negative sign so that the equation was
\frac 0.20 V {-0.0296} = log \frac [0.5] {[Fe²⁺]} (trying to use LaTeX, not sure how well this will turn out...)
and I got a much more reasonable answer!
 

Title: Re: Calculating Concentration from Cell Potential
Post by: chanelo on March 10, 2013, 02:31:41 PM

Ok obviously I'm not doing LaTeX right lol sorry about that, I'll keep trying

Title: Re: Calculating Concentration from Cell Potential
Post by: Borek on March 10, 2013, 02:56:50 PM

You need to surround LaTeX code with [tеx][/tеx] tags.

There are also some other problems, but that's the first thing to correct.

Title: Re: Calculating Concentration from Cell Potential
Post by: chanelo on March 10, 2013, 05:51:32 PM

Other problems with LaTeX or the equation?

Title: Re: Calculating Concentration from Cell Potential
Post by: Borek on March 10, 2013, 06:45:34 PM

I didn't like the result while attempting the problem yesterday, but I don't have my calculator here at the moment and I can't check what the problem was. So: it is possible something is still wrong.