[hallie3]

Determine Ecell (in V) at 298.15 K for a cell composed of a platinum electrode in a mixture of 0.31 M Sn2+and 0.457 M Sn4+ coupled to a platinum electrode in a solution where the [H+] is 0.408 M and the partial pressure of H2 is 1.667 atm.
(From a previous part to this question, the E cell of Sn4+(aq) + 2e− → Sn2+(aq) was found (marked correct) to be: 0.151 V.)

My attempt:
2H+(aq) + 2e- → H2(g)
E_H= 0.000 + (0.0592/2) x log((0.408)²/1.667)
E_H= -0.0296 V

Sn4+(aq) + 2e− → Sn2+(aq)
E_Sn= 0.151 + (0.0592/2) x log(0.31/0.457)
E_Sn= 0.146 V



E_cell= E_cathode-E_anode
E_cell=0.146-(-0.0296)
E_cell=0.176 V

This is being marked as incorrect. Where have I gone off? Am I misunderstanding the question in some way, or is my method flawed? Thank you very much in advance!

[Radu]

  at E_{Sn²⁺/Sn⁴⁺}, you wrote 0.059/2*lg( [Sn²⁺]/[Sn⁴⁺], when it actually is 0.059/2*lg( [Sn⁴⁺]/[Sn²⁺].