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Homework Help: [Chemistry] Another electrochemistry question

  1. Apr 1, 2007 #1
    1. The problem statement, all variables and given/known data

    Given the following half-reactions:
    Ce4+ + e− → Ce3+ E° = 1.72 V
    Fe3+ + e− → Fe2+ E° = 0.771 V

    A solution is prepared by mixing 7.0 mL of 0.30 M Fe2+ with 8.0 mL of 0.12 M Ce4+.

    Calculate [Ce4+] in the solution.

    I've got the potential of a platinum electrode dipped into the resulting, equilibrated, solution (relative to SHE) to be 0.767 V, and keq of 1.1×10^16 from this equation: Ce4+ + Fe2+ ⇌ Ce3+ + Fe3+.

    How do you get [Ce4+]? Why isn't that the same as the concentration posted in the question?

    Thanks!
     
  2. jcsd
  3. Apr 1, 2007 #2
    R)____Ce4+___ +_________ Fe2+___⇌___Ce3+___+_Fe3+
    I) .008*.12/.015 ______.007*.3/.015______0_________0
    C)_____-x _______________-x ___________x ________x
    E) .008*.12/.015 -x ____.007*.3/.015 _____x ________x

    [tex]k=\frac{x*x}{(\frac{0.008*0.12}{0.015}-x)(\frac{0.007*0.3}{0.015}-x)}[/tex]
     
  4. Apr 1, 2007 #3
    but since the k is such large number, [Ce4+] is going to be large too right?
     
  5. Apr 3, 2007 #4
    no it will be near 0. X represents Ce3+ concentration and that number minus x represent the Ce4+ Solve x using quadratic. Subtract from that number. Then Ce4+ ends being a pretty small number
     
  6. Apr 4, 2007 #5
    so in this case, k is 1.1×10^16? cause when i try the quadratic on the internet quadratic solver, it says the answer is an imagary number....
     
  7. Nov 15, 2007 #6
    i got the same problem too.... i just cannot solve the quadratic equation
     
  8. Nov 15, 2007 #7
    I used this method too, but i did not get the correct answer

    after i found X, it is the [Ce3+], i subtract it from 0.008*0.12/0.015 which is the [Ce4+] at equilibrium
    my answer was very close to 0...around 10^-8
    why am i not getting the right answer
     
  9. Mar 25, 2008 #8
    Can anyone explain how did this person get potential of a platinum electrode?
     
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