VKC ScienceChemistry for written exams Telegram

UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper I

Q4(b). Mean molar heat of vaporisation of water from two vapour pressures

Question

The vapour pressure of water at 363.2 K is 529 torr. Use the Clausius–Clapeyron equation to determine the average value of molar heat of vaporization, ΔH, of water between 363.2 K and 373.2 K. [R = 8.314 J mol⁻¹ K⁻¹]

Answer in brief

ΔHvap = R ln(P₂/P₁)/(1/T₁ − 1/T₂) = 40.8 kJ mol⁻¹, taking P₂ = 760 torr at the normal boiling point 373.2 K.

Part of VKC Basic

Read the key points and the full model answer with VKC Basic

VKC Basic (₹999 for 6 months) opens every solved answer, the notes, the handbook and the daily answer archive. Every Mains test series includes it.

See what is included Already a member? Sign in

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.

The handwritten model answer pages open with the VKC plan.
Read the whole 2022 solution2022 question paper

Topic notes: the phase rule, phase diagrams, the Clausius–Clapeyron equation, eutectics and partially miscible liquids, with every Civil Services, IFoS and Geo-Scientist PYQ →

CoversClausius–Clapeyron equationenthalpy of vaporisationvapour pressureTrouton's rulenormal boiling point

All parts of Question 4