\"Calculate the potential for the titration of 50 mL of 0.1 M Fe2+ in 1M Hal wit

Question

"Calculate the potential for the titration of 50 mL of 0.1 M Fe2+ in 1M Hal with 0.25 M MnO4- at the following volumes: at the half-way point, at the equivalence point, and 20 mL past the eq point. The indicator electrode is a platinum wire and the reference electrode is a saturated calomel electrode (SCE). E(sce)=0.241 V."

For this question, I've only gotten as far as determining the correct Nernst equations and calculating the equivalence point. The half-way point is 100 mL of MnO4-, but I have no idea where to go from there. Any help would be greatly appreciated.

Explanation / Answer

Titration

Nernst equation,

E = {0.68 - 0.0592 log([Fe2+]/[Fe3+]} - 0.241

E = {1.507 - 0.0592/5 log([Mn2+]/[MnO4^-][H+]^8} - 0.241

(a) at half-way point

[Fe2+] = [Fe3+]

E = 0.68 - 0.241 = 0.439 V

(b) At equivalence point

6E = {8.215 - 0.0592 log(1/[H+]^8} - 0.241

with [H+] = 1 M

E = 1.369 - 0.241 = 1.128 V

(c) 20 ml past equivalence point

E = {1.507 - 0.0592/5 log([Mn2+]/[MnO4^-[H+]^8} - 0.241

[Mn2+]/[MnO4^-] = 20/1, [H+] = 1 M

E = {1.507 - 0.0592/5 log(20)} - 0.241

   = 1.25 V

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