UPSC Combined Geo-Scientist, Mains · 2023 · Mains Paper II
Q1(j). pH from a calomel–quinhydrone cell
Question
Calculate the pH of the solution containing a mixture of quinone and hydroquinone (1 : 1) in the cell SCE | solution | H₂Q, Q, Pt. The measured EMF of the cell is 0.221 V at 298 K; E°SCE = 0.242 V and E°H₂Q,Q = 0.699 V.
Answer in brief
Ecell = 0.457 − 0.0591 pH = 0.221 V, so pH = 3.99 ≈ 4.0.
Key points
- 0.242 V is the potential of the saturated calomel electrode at 25 °C (not a true standard potential); used as printed.
- Right electrode: Q + 2H⁺ + 2e⁻ → H₂Q; with [Q] = [H₂Q], EQ = E°Q − 0.0591 pH.
- Ecell = Eright − Eleft = (0.699 − 0.0591 pH) − 0.242 = 0.457 − 0.0591 pH.
- 0.221 = 0.457 − 0.05913 pH ⇒ pH = 0.236/0.05913 = 3.99.
- Valid only for pH below about 8: in alkaline solution H₂Q ionises and is oxidised by air.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversquinhydrone electrodepH measurementsaturated calomel electrodeNernst equationemf of cell
All parts of Question 1
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- 1(b) Exceptional behaviour of hydrogen and helium5 marks
- 1(c) Unit cell of solid C₆₀ and its carbon atoms5 marks
- 1(d) Planes that diffract: simple cubic, a = 5 Å5 marks
- 1(e) Chemical potential and its link with Gibbs energy5 marks
- 1(f) Debye–Hückel limiting law for γ±5 marks
- 1(g) Activation energy of 2HI → H₂ + I₂5 marks
- 1(h) Best coagulant for the Sb₂S₃ sol5 marks
- 1(i) E° of Na⁺/Na from ΔrH° and ΔrS°5 marks
- 1(j) pH from a calomel–quinhydrone cell5 marks
- 1(k) Degeneracy of the level 6h²/8mL² in a cubic box5 marks
- 1(l) Is Peⁱᵏˣ + Qe⁻ⁱᵏˣ an eigenfunction?5 marks
- 1(m) Zero-point energy in joule and in cm⁻¹5 marks
- 1(n) Microwave activity of SF₆, CO, H₂, H₂O and CO₂5 marks
- 1(o) Stark–Einstein law and the quantum yield5 marks
- 1(p) Decay routes of a photoexcited molecule5 marks