Given the reaction Cr2O72-(aq) + I-(aq) Cr3+(aq) + I2 (s) in acidic solution, ca

Question

Given the reaction

Cr2O72-(aq) + I-(aq) Cr3+(aq) + I2 (s)

in acidic solution, calculate the emf of a cell operating at 25°C when [Cr2O72-] = 0.020 M,[I-] = 0.015M, [Cr3+] = 0.40 M, and [H+] = 0.50 M. Provide an answer correct to 2 significant figures if your answer is a 2 digit number and to three sig figs if your answer is a 3 digit number. For example, if you believe the correct answer is +0.66V, simply report .66. If you believe that the correct answer is -1.01V , simply report -1.01. Do not answer this question using scientific notation. Do not enter a positive sign if your answer is a positive number. Do not provide units for your answer.
(hint: emf and cell potential are the same thing. Standard emf and standard cell potential are the same thing.)

Explanation / Answer

For the given electrochemical reaction,

balanced equation,

Cr2O7^2- + 6I- + 14H+ --> 2Cr3+ + 3I2 + 7H2O

using Nernst equation,

E = Eo - 0.0592/n logK

taking values from above,

E = (1.33 - 0.54) - 0.0592/6 log[(0.4)^2/(0.02)(0.5)^14]

   = .74

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