UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II
Q1(o). Measuring light intensity with a chemical actinometer
Question
Explain briefly about measuring the intensity of radiation using a chemical actinometer.
Answer in brief
Run a photoreaction of known quantum yield ϕ. The moles n reacted in time t give Iabs = n/(ϕt).
Key points
- Chemical actinometer: a photochemical reaction whose quantum yield is accurately known.
- Irradiate for time t, estimate n: Iabs = n/(ϕt) einstein s⁻¹ and I₀ = Iabs/(1 − 10⁻ᴬ).
- Ferrioxalate: 2[Fe(C₂O₄)₃]³⁻ → 2Fe²⁺ + 5C₂O₄²⁻ + 2CO₂, ϕ ≈ 1.2 (254 to 366 nm); Fe²⁺ read as its 1,10-phenanthroline complex at 510 nm.
- Uranyl oxalate: H₂C₂O₄ → CO₂ + CO + H₂O, ϕ ≈ 0.5 to 0.6. Oxalic acid left is titrated with KMnO₄.
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.
Coverschemical actinometerquantum yieldferrioxalate actinometeruranyl oxalate actinometerintensity of radiation
All parts of Question 1
- 1(a) Why Z > 1 for H₂ and He at all pressures5 marks
- 1(b) Corresponding state of H₂S for N₂ at 25 °C and 1 atm5 marks
- 1(c) Band structure of n-type and p-type semiconductors5 marks
- 1(d) Symmetry elements of orthorhombic Mg₂SiO₄5 marks
- 1(e) Fall of G with temperature at constant pressure5 marks
- 1(f) Z > 1 and f > P for the gas P(Vm − b) = RT5 marks
- 1(g) Overall equation and the role of each species5 marks
- 1(h) log k against pH for specific H⁺ catalysis5 marks
- 1(i) KCl salt bridge for a silver–lead cell5 marks
- 1(j) Equilibrium constant from standard potentials5 marks
- 1(k) Conditions for an acceptable wave function5 marks
- 1(l) No solution with E = 0 for the particle in a box5 marks
- 1(m) Discrete levels and the resonance condition5 marks
- 1(n) Rotational activity of CO and CO₂5 marks
- 1(o) Measuring light intensity with a chemical actinometer5 marks
- 1(p) Why watch hands glow in the dark5 marks