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For the cell Zn(s) + Cu²+(1.0 M) -> Cu(s) + Zn²+(0.1 M), the measured e.m.f. at 25 deg C is 1.3 V. What is the standard cell potential E_cell^o for this reaction?

1. 1.27 V
2. 1.33 V
3. 1.10 V
4. 1.40 V

Correct answer: 1.27 V

Solution

The Nernst equation gives E = E^o - (0.0591/n) log Q. The reaction consumes Cu²+ and produces Zn²+, so Q = [Zn²+]/[Cu²+]. Since Q < 1 (0.1), the log term is negative, which makes the measured E larger than E^o. So E^o must be smaller than 1.3 V.

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