Physical Techniques in Inorganic Chemistry: 30 Multiple Choice Questions with Explanations

1. Which of the following is not listed as a type of data provided by physical techniques in inorganic chemistry? a) Molecular structure b) Reaction mechanisms c) Macroscopic density d) Magnetic propertiesAnswer: c) Macroscopic density Explanation: The “Introduction” explicitly lists molecular structure, bonding characteristics, electronic configuration, magnetic properties, and dynamic processes (reaction mechanisms) as data provided. Macroscopic density, while a physical property, is not highlighted as a primary output of these microscopic techniques in the given text.
2. X-ray Diffraction (XRD) is considered the “gold standard” for determining the structure of what type of material? a) Amorphous solids b) Liquid solutions c) Crystalline solids d) GasesAnswer: c) Crystalline solids Explanation: The “X-ray Diffraction (XRD)” section’s principle states it “stands as the undisputed premier technique for definitively determining the precise three-dimensional atomic and molecular structure of crystalline solids.”
3. What is the typical wavelength range for X-rays used in XRD? a) 200-800 nm b) 0.5-2.5 Å c) 4000-400 cm⁻¹ d) Radiofrequency rangeAnswer: b) 0.5-2.5 Å Explanation: Under “Principle” of XRD: “X-rays are chosen because their wavelengths (typically in the range of 0.5 to 2.5 Å, or 50 to 250 picometers) are precisely comparable to the interatomic distances…”
4. In Bragg’s Law (nλ=2dsinθ), what does d represent? a) The order of diffraction. b) The wavelength of X-rays. c) The interplanar spacing. d) The Bragg angle.Answer: c) The interplanar spacing. Explanation: In the “Bragg’s Law” section, d is defined as “The interplanar spacing, or the distance between successive, parallel planes of atoms…”
5. Which type of XRD technique is used for polycrystalline samples and produces a diffractogram? a) Single-Crystal XRD (SC-XRD) b) Powder XRD (PXRD) c) Neutron Diffraction d) Electron DiffractionAnswer: b) Powder XRD (PXRD) Explanation: Under “Instrumentation” -> “Powder XRD (PXRD)”: “This technique is employed for polycrystalline samples… The output is a diffractogram…”
6. What is a key application of Single-Crystal XRD (SC-XRD)? a) Quantitative analysis of liquid concentrations. b) Phase identification of amorphous materials. c) Absolute structure determination at the atomic level. d) Studying reaction kinetics in the gas phase.Answer: c) Absolute structure determination at the atomic level. Explanation: Under “Applications” -> “Absolute Structure Determination”: “SC-XRD is the definitive, gold-standard method for solving the precise three-dimensional structure of newly synthesized compounds…”
7. A significant limitation of XRD is that it has difficulty precisely locating which type of atom? a) Heavy atoms (e.g., metals) b) Halogen atoms (e.g., Cl, Br) c) Light atoms (e.g., Hydrogen) d) Transition metalsAnswer: c) Light atoms (e.g., Hydrogen) Explanation: Under “Limitations” of XRD: “Light atoms, particularly hydrogen, have very few electrons and thus scatter X-rays weakly, making them difficult to locate precisely.”
8. For a vibrational mode to be IR active, what must occur during the vibration? a) A change in the molecule’s polarizability. b) A net change in the molecule’s dipole moment. c) Emission of photons. d) Scattering of light.Answer: b) A net change in the molecule’s dipole moment. Explanation: Under “Infrared (IR) Spectroscopy” -> “Mechanism (Dipole Moment Change)”: “For a specific vibrational mode to be IR active… it must cause a net change in the molecule’s electric dipole moment during the vibration.”
9. Raman spectroscopy is based on what phenomenon? a) Absorption of IR radiation. b) Elastic scattering of monochromatic light. c) Inelastic scattering of monochromatic light. d) Emission of UV light.Answer: c) Inelastic scattering of monochromatic light. Explanation: Under “Raman Spectroscopy” -> “Mechanism (Polarizability Change)”: “Raman spectroscopy is based on the inelastic scattering of monochromatic light…”
10. What is the Rule of Mutual Exclusion used for in IR and Raman spectroscopy? a) To determine bond lengths. b) To identify functional groups. c) To experimentally determine molecular symmetry. d) To quantify reaction rates.Answer: c) To experimentally determine molecular symmetry. Explanation: Under “Complementarity of IR and Raman: Symmetry Probes” -> “Rule of Mutual Exclusion”: “This rule is exceptionally powerful for experimentally determining the molecular symmetry of unknown compounds…”
11. What is a limitation of IR spectroscopy related to aqueous solutions? a) Fluorescence from samples. b) Strong absorption by water can interfere with measurements. c) It requires very low temperatures. d) It only detects non-polar bonds.Answer: b) Strong absorption by water can interfere with measurements. Explanation: Under “Limitations” of Vibrational Spectroscopy -> “IR”: “Strong absorption by water (which has a broad O-H stretch) can significantly interfere with measurements in aqueous solutions…”
12. In UV-Vis spectroscopy, what phenomenon causes the characteristic vibrant colors of transition metal complexes? a) Charge Transfer (CT) Transitions b) Ligand-Centered Transitions c) d-d Transitions (Ligand Field Transitions) d) Spin-Spin CouplingAnswer: c) d-d Transitions (Ligand Field Transitions) Explanation: Under “Electronic Spectroscopy” -> “Mechanism of Absorption” -> “d-d Transitions”: “these are the most common and important type of electronic transitions responsible for their characteristic vibrant colors.”
13. Which type of electronic transition is typically much more intense (higher molar absorptivity) than d-d transitions? a) d-d Transitions b) Charge Transfer (CT) Transitions c) Ligand-Centered Transitions d) Vibrational TransitionsAnswer: b) Charge Transfer (CT) Transitions Explanation: Under “Charge Transfer (CT) Transitions”: “They are typically much more intense (higher molar absorptivity, ϵ) than d-d transitions…”
14. The intensely purple color of the permanganate ion (MnO4−​) is a classic example of which type of transition? a) d-d transition b) MLCT transition c) LMCT transition d) π→π∗ transitionAnswer: c) LMCT transition Explanation: Under “Ligand-to-Metal Charge Transfer (LMCT)”: “Example: The intensely purple color of the permanganate ion (MnO4−​)… arises from LMCT from oxygen orbitals to manganese.”
15. What law relates absorbance to molar absorptivity, path length, and concentration in UV-Vis spectroscopy? a) Bragg’s Law b) Beer-Lambert Law c) Hund’s Rule d) Pauli Exclusion PrincipleAnswer: b) Beer-Lambert Law Explanation: Under “Quantitative Analysis (Beer-Lambert Law)”: “The Beer-Lambert Law (A=ϵbc) relates absorbance (A) to molar absorptivity (ϵ), path length (b), and concentration (c).”
16. What is a limitation of UV-Vis spectroscopy related to d-d transitions? a) They are usually very high intensity. b) They only occur in d0 or d10 complexes. c) They are inherently “forbidden” by quantum mechanical selection rules, leading to low intensity. d) They cannot be used to determine Δ.Answer: c) They are inherently “forbidden” by quantum mechanical selection rules, leading to low intensity. Explanation: Under “Limitations” of Electronic Spectroscopy -> “Weak d-d Transitions”: “d-d transitions are inherently “forbidden”… allowing transitions to occur, but they remain relatively low intensity…”
17. What type of substance is weakly attracted into a magnetic field and possesses one or more unpaired electrons? a) Diamagnetic b) Ferromagnetic c) Paramagnetic d) AntiferromagneticAnswer: c) Paramagnetic Explanation: The “Paramagnetism” section defines it: “Exhibited by substances that possess one or more unpaired electrons… Paramagnetic substances are weakly attracted into a magnetic field.”
18. What is the fundamental property that causes diamagnetic substances to be weakly repelled by a magnetic field? a) Presence of unpaired electrons. b) High positive charge. c) All electrons are paired. d) Their large size.Answer: c) All electrons are paired. Explanation: The “Diamagnetism” section states: “Exhibited by substances in which all electrons are paired.”
19. What is the formula for the spin-only magnetic moment (μspin−only​)? a) n(n+2) BM b) n(n−2)​ BM c) n(n+2)​ BM d) n/(n+2) BMAnswer: c) n(n+2)​ BM Explanation: The “Calculation of Magnetic Moment” section provides this formula.
20. What is a crucial application of magnetic susceptibility measurements for transition metal complexes with certain d-electron counts? a) Determining molecular weight. b) Determining precise bond lengths. c) Determining d-Electron Configuration and Spin State (high vs. low spin). d) Observing vibrational modes.Answer: c) Determining d-Electron Configuration and Spin State (high vs. low spin). Explanation: Under “Applications” of Magnetic Susceptibility: “This is the most crucial application… for definitively determining which spin state is adopted.”
21. Which limitation of magnetic susceptibility measurements notes that measured μeff​ can be higher than spin-only values due to orbital contributions? a) Complexity b) Diamagnetic Corrections c) Spin-Only Approximation d) Temperature DependenceAnswer: c) Spin-Only Approximation Explanation: Under “Limitations” -> “Spin-Only Approximation”: “there can be significant contributions from orbital angular momentum, which causes the measured μeff​ to be higher than the spin-only value.”
22. Which atomic nuclei are typically NMR active? a) All nuclei regardless of spin. b) Only nuclei with a non-zero spin. c) Only nuclei with even mass numbers. d) Only nuclei with a negative charge.Answer: b) Only nuclei with a non-zero spin. Explanation: The “Principle” of NMR states: “Only nuclei with a non-zero nuclear spin (e.g., 1H,13C,31P,19F,15N,195Pt, but not 12C or 16O) are NMR active.”
23. What does “chemical shift (δ)” in NMR provide information about? a) The number of neighboring nuclei. b) The exact mass of the nucleus. c) The electronic shielding or deshielding of the nucleus. d) The vibrational frequency of the molecule.Answer: c) The electronic shielding or deshielding of the nucleus. Explanation: Under “Chemical Shift (δ)”: “It provides direct information about the electronic shielding or deshielding experienced by the nucleus…”
24. What information does the “coupling constant (J)” in NMR provide? a) Molecular weight and elemental composition. b) The strength of the external magnetic field. c) Connectivity and bonding pathways between neighboring nuclei. d) The rate of electron spin relaxation.Answer: c) Connectivity and bonding pathways between neighboring nuclei. Explanation: Under “Spin-Spin Coupling (J)”: “The J value provides crucial information about: Connectivity… Bonding Pathway…”
25. What is a key application of NMR spectroscopy in inorganic chemistry? a) Determining the crystal structure of amorphous solids. b) Quantifying the number of unpaired electrons. c) Studying the rates and mechanisms of ligand exchange reactions. d) Identifying highly colored charge transfer complexes.Answer: c) Studying the rates and mechanisms of ligand exchange reactions. Explanation: Under “Applications” -> “Ligand Exchange and Dynamics”: “NMR can be used to study the rates and mechanisms of ligand exchange reactions…”
26. A limitation of NMR spectroscopy is its relatively low sensitivity, requiring: a) Gaseous samples. b) Extremely pure samples. c) Very low temperatures. d) Concentrated samples (mg to gram quantities).Answer: d) Concentrated samples (mg to gram quantities). Explanation: Under “Limitations” of NMR -> “Sensitivity”: “NMR is relatively insensitive… requiring concentrated samples (typically mg to gram quantities).”
27. EPR spectroscopy is specifically used to study systems that contain: a) All electrons paired. b) Only nuclei with non-zero spin. c) Unpaired electrons. d) Fully ionized species.Answer: c) Unpaired electrons. Explanation: The “Principle” of EPR states: “EPR… is a specialized spectroscopic technique exclusively used to study systems that contain unpaired electrons.”
28. What does “hyperfine coupling” in EPR spectroscopy provide information about? a) The overall molecular symmetry. b) The electron spin density distribution onto ligands. c) The bond lengths in the complex. d) The strength of the external magnetic field.Answer: b) The electron spin density distribution onto ligands. Explanation: Under “Hyperfine Coupling”: “Crucially, it reveals the extent to which the unpaired electron’s spin density is delocalized onto the ligands or other atoms in the complex. This provides direct experimental evidence for covalent character in metal-ligand bonds.”
29. What is a key application of EPR in inorganic chemistry related to reaction mechanisms? a) Determining molecular weight of products. b) Identifying non-radical intermediates. c) Detecting and characterizing short-lived radical species. d) Balancing redox equations.Answer: c) Detecting and characterizing short-lived radical species. Explanation: Under “Applications” -> “Studying Reaction Intermediates and Mechanisms”: “EPR is a powerful tool for detecting and characterizing short-lived radical species generated during chemical reactions…”
30. In Mass Spectrometry (MS), what is measured? a) The wavelength of absorbed light. b) The mass-to-charge ratio (m/z) of ions. c) The number of unpaired electrons. d) The vibrational frequencies of molecules.Answer: b) The mass-to-charge ratio (m/z) of ions. Explanation: The “Principle” of MS states: “Mass spectrometry… measures the mass-to-charge ratio (m/z) of ions.”