Homework Help: Chemistry

Posted by Alanna on Tuesday, April 3, 2012 at 10:59pm.

The following voltaic cell registers an E cell = 0.100V.

Pt|H2(g 1 atm)|H^+(x M)||H^+(1.0 M)|H2(g 1 atm)

What is the pH of the unknown solution?

I am completely baffled as I was not aware until I read this question that pH could be found from voltaic cells. Any help would be greatly appreciated.

Chemistry - DrBob222, Tuesday, April 3, 2012 at 11:23pm
The anode is H2 ==> 2H^+ + 2e
The cathode is 2H^+ + 2e ==> H2
--------------------------------
The cell is H2+2H^+==>2H^+ + H2

Ecell = Eocell -(0.0592/2)logQ
log Q = (H^+)^2*pH2/(H^+)^2*pH2
H^+ in numerator is the unknown; H^+ in denominator is 1M. pH2 is 1atm in both cases. Solve for (H^+) and convert to pH. Ecell is 0.1.
Chemistry - Alanna, Tuesday, April 3, 2012 at 11:37pm
How am I able to solve for [H^+] when I don't have Q and can't rearrange?
Chemistry - Alanna, Tuesday, April 3, 2012 at 11:48pm
Ecell=E*cell-0.0592/2*log[xM]/[1.0]

-3.378 = log[xM]/[1.0M]

x=0.0000410 M

pH=3.38

What did I do wrong because this is not the right answer
Chemistry - DrBob222, Wednesday, April 4, 2012 at 12:09am
-3.378 is right but x is not.
First, -3.378 = log X^2
4.19E-4 = x^2 and not E-5 as you have.
Second, you didn't take the square root.
Third, if 4.10E-5 were the right answer, -log of that for pH is not right.
So -3.378 = log x^2 and solve for x, then convert to pH.

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