What pressure (bar) of H2 would be required to make emf of hydrogen electrode zero in pure water at 25 ?
- 0.5
- 10-14
- 1
- 10-7
Answer
(2) 10-14
Solution
2H+(aq) + 2e- H2(g), E = 0
Nernst: E = E - 0.0592 PH2[H+]2
E = 0 ⇒ PH2[H+]2 = 0 ⇒ PH2 = [H+]2
∵ pure water at 25 : [H+] = 10-7 M
⇒ PH2 = (10-7)2 = 10-14 bar
Answer: (2)
Note: The official NTA answer key gives option (3). Our faculty team has re-checked this question and finds option (2) correct, as explained above.