JEE Main 2024 · 4 April · morning · Single correct · +4 / −1
What pressure (bar) of H₂ would be required to make emf of hydrogen electrode zero in pure water at 25° C ?
- (a) 0.5
- (b) 10⁻¹⁴
- (c) 1
- (d) 10⁻⁷
Answer and solution
Answer (b) 10⁻¹⁴
Solution
Chemistry ma’am · 3 steps
Write the Nernst equation for the hydrogen electrode and set it to zero; [H⁺] comes in squared.
- Electrode reaction. The hydrogen electrode works by 2H⁺ + 2e⁻ → H₂, so n = 2 and E° = 0. In pure water at 25°C, [H⁺] = 10⁻⁷ M.
E = E° - 0.0591/2 log p_(H₂)/[H⁺]²
- Set E to zero. With E = 0 and E° = 0, the log term must be zero. So p_(H₂)/[H⁺]² = 1, which means p_(H₂) = [H⁺]².
- So. p_(H₂) = (10⁻⁷)² = 10⁻¹⁴ bar, which is option (b).
The idea to keep. For any hydrogen electrode, E = 0 exactly when p_(H₂) = [H⁺]². Always balance the half-reaction first, so the powers in the Nernst equation come out right.
Answer from the NTA final answer key. Solution by PrepWiser.