AP Chemistry Question 212: Answer and Explanation

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Question: 212

6. The molar heat of vaporization of water is +43.9 kJ. What is the entropy change for the vaporization of water?

  • A. 8.49 J mol-1 K-1
  • B. 4.184 J mol-1 K-1
  • C. 2.78 J mol-1 K-1
  • D. 118 J mol-1 K-1

Correct Answer: D


(D) The transformation of water from H2O(l) ↔ H2O(g) at 100 °C has ΔG° = 0. Therefore ΔH° = T ΔS°. Because we know the heat of vaporization and the boiling point of water, 100 °C (373 K), the entropy change can be calculated as