Q46 · CSIR-NET Chemistry, December 2019

Paper: CSIR-NET December 2019 · Subject: Physical Chemistry · Chapter: Molecular Spectroscopy · Topic: Molecular Spectroscopy – General · Marks: 2 · Difficulty: Easy

The atomic masses of fluorine and hydrogen are 19.0 and 1.0 amu, respectively ($1\,\mathrm{amu}=1.67\times10^{-27}\,\mathrm{kg}$). The bond length of HF is $2.0\,\AA$. The moment of inertia of HF is
(a)$3.2 \times 10^{-47} \mathrm{kg} \mathrm{m}^{2}$
(b)$6.4\times10^{-47}\,\mathrm{kg\,m^{2}}$
(c)$9.6\times10^{-47}\,\mathrm{kg\,m^{2}}$
(d)$4.8\times10^{-47}\,\mathrm{kg\,m^{2}}$
Answer
Answer: B ✓ checked by 4AB · confidence high
Explanation
Reduced mass of HF: μ = (19 × 1)/(19 + 1) amu = 0.95 × 1.67×10⁻²⁷ kg = 1.587×10⁻²⁷ kg. I = μr² = 1.587×10⁻²⁷ × (2.0×10⁻¹⁰ m)² = 6.35×10⁻⁴⁷ ≈ 6.4×10⁻⁴⁷ kg m². Answer: (B).

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