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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 · 2023 · Paper I

Q4(a). van der Waals constant b, compression factor, interactions

Question

The molar volume of a certain gas is found to be 5.0 × 10⁻⁴ m³ mol⁻¹ at 273 K and 3.0 × 10⁶ Pa. This gas obeys the van der Waals equation with a = 0.50 m⁶ Pa mol⁻². (i) Calculate the other van der Waals constant, b. (ii) What is the compression factor for this gas at the same temperature and pressure? (iii) Comment on the nature of the molecular interactions of this gas.

Answer in brief

(i) b = 4.6 × 10⁻⁵ m³ mol⁻¹; (ii) Z = 0.66; (iii) Z < 1, so attractive forces dominate.

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Coversvan der Waals equationcompression factorreal gasBoyle temperaturesecond virial coefficient

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