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Homework Help: Redox equilibrium

  1. Aug 10, 2013 #1
    1. The problem statement, all variables and given/known data

    I.) Give an estimate for the stability constant of the complex [FeCl]2+, given that the standard redox potential for the Fe3+/Fe2+ system in 1 moldm-3 HCl is 0.710 V.

    The "standard electrode potential of the reaction Fe3+ + e- -> Fe2+" (quote from the question) had previously been calculated as 0.772 V but I myself cannot see how this should be different just because 1 moldm-3 HCl is present.

    IV.) H3AsO4 and K4Fe(CN)6 are dissolved in water in a stoichiometric ratio. What will the ratio of [H3AsO4]/[H3AsO3] be at equilibrium if pH = 2.00 is maintained?

    V.) Are the following equilibrium concentrations possible in an aqueous solution? If yes, calculate the pH of the solution. [H3AsO4]=[H3AsO3]=[I3-]=[I-]=0.100 M (where on earth does the I3- come from?!)

    Available data: [Fe(CN)6]3-/[Fe(CN)6]4- has E° = +0.356 V. H3AsO4/H3AsO3 has E° = +0.560 V. I2/2 I- has E° = +0.540 V.

    2. Relevant equations

    ΔG°(rxn) = -n(rxn) * F * E°(rxn) = - R * T * ln(K(rxn))

    3. The attempt at a solution

    My thinking was that we need the value of K for reaction H3ASO4 + 2H+ + 2e- -> H3AsO3 + H2O. This can easily be found from the value of the standard electrode potential for this reaction itself. Then take the inverse, before dividing by [H+]2=10-4. I got 1.17*10-15, but the correct answer is 0.107. For the next question, I soon as I saw I3- instead of I2 I was defeated.
  2. jcsd
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