VKC ScienceChemistry for written exams Telegram

UPSC Combined Geo-Scientist

Chemist

  • 3 descriptive papers
  • 200 marks each
  • 3 h per paper

Mains Paper II covers physical chemistry.

UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II

Q1(b). Corresponding state of H₂S for N₂ at 25 °C and 1 atm

Question

Calculate the temperature and pressure of 1 mole of H₂S which will correspond to the state of 1 mole of N₂ at 25 °C and 1.0 atm pressure. Given: N₂, Pc = 33.5 atm, Tc = 126.3 K; H₂S, Pc = 88.3 atm, Tc = 373.2 K.

Answer in brief

H₂S at T = 881 K (608 °C) and P = 2.64 atm.

Key points

  • Law: gases at the same reduced temperature and reduced pressure are in corresponding states.
  • N₂: Tr = 298.15/126.3 = 2.361, Pr = 1.0/33.5 = 0.0299.
  • H₂S: T = Tr Tc = 2.361 × 373.2 = 881 K.
  • H₂S: P = Pr Pc = 0.0299 × 88.3 = 2.64 atm.

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.

Model answer to Q1(b), 2025 Mains Paper II: solution pages 3 and 4
Solution pages 3 and 4 of 33
Read the whole 2025 solution2025 question paper

Topic notes: real gases, the van der Waals equation, critical constants and Boyle temperature, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverslaw of corresponding statesreduced temperaturereduced pressurecritical constantsH2S

All parts of Question 1