Q3(b). Avogadro constant from diffraction
The density of LiF is 2.601 g cm⁻³. The (111) first-order reflection in the X-ray diffraction from LiF occurs at 8°44′ when X-rays of wavelength 70.8 pm are used. If there are four LiF molecules per unit cell, calculate Avogadro's number. LiF crystallizes in the cubic system. Li = 6.939, F = 18.998.
Model answer
The free solution for 2023 answers questions 1, 2, 4, 5 and 6, the ones picked in its time plan. Question 3 is answered in full in the IFoS Chemistry Paper I, 2021 to 2025 book, with every part of every paper.
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UPSC Indian Forest Service (Main) Chemistry Paper I: Model Answers 2021 to 2025
- All eight questions of each paper, with the question printed above its answer
- Physical chemistry numericals worked with units; inorganic structures drawn
- Topic-wise weightage and a topic-wise index
CoversBragg's lawAvogadro numberX-ray diffractionLiFMiller indices (111)
All parts of Question 3
- 3(a) Ionic distances and radius-ratio predictions10 marks
- 3(b) Avogadro constant from diffraction10 marks
- 3(c) Ideal and van der Waals pressures10 marks
- 3(d) Pressure as a state function10 marks