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For the cell reaction Zn(s) + 2H+(aq) -> Zn2+(0.1 M) + H2(g, 1 atm), the measured emf at 25 deg C is 0.28 V. Using E°(Zn2+/Zn) = -0.76 V and E°(H+/H2) = 0, determine the pH of the solution surrounding the hydrogen electrode.

1. 8.62
2. 10.0
3. 7.50
4. 5.30

Correct answer: 8.62

Solution

The reaction uses 2 electrons. E°cell = 0.76 V. The reaction quotient Q = [Zn2+]*pH2 / [H+]². Plugging into the Nernst equation and solving for [H+] gives a very small concentration, corresponding to a basic/near-neutral pH around 8.6. This is a classic Nernst-equation pH determination.

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