Indian Forest Service (Mains), Chemistry optional · 2025 · Paper I
Q3(a)(ii). NaCl unit cell and Avogadro constant
Question
Describe the NaCl unit cell and calculate Avogadro’s constant from density 2.165 g/cm³, Na–Cl distance 281 pm and molar mass 58.5 g/mol.
Answer in brief
NaCl consists of two interpenetrating FCC sublattices, with 6:6 coordination and four formula units per cell; the data give NA ≃ 6.09 × 10²³ mol⁻¹.
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CoversNaCl unit cellrock-salt structureAvogadro constant from densityFCCcoordination number 6:6
All parts of Question 3
- 3(a)(i) BCC edge length from diffraction5 marks
- 3(a)(ii) NaCl unit cell and Avogadro constant10 marks
- 3(b)(i) Composition after hydrogen leakage5 marks
- 3(b)(ii) Entropy change along two paths10 marks
- 3(c) Three-state nuclear-spin partition function10 marks