Dipole Moment

Answer: B. Dipole moment μ = q × d. Charge (C) × distance (m) = C·m. The Debye is a non-SI unit commonly used in chemistry (NCERT Class 11 Chemistry Chapter 4, page 111).

Why A is wrong: A is the commonly used unit in chemistry but is not the SI unit. 1 D = 3.336 × 10⁻³⁰ C·m.

Why C is wrong: C has the wrong operation — dipole moment is charge multiplied by distance, not divided.

Why D is wrong: D (N·m) is the unit of torque, not dipole moment. Confusing force-related and charge-related quantities.

Answer: A. CCl₄ has a symmetric tetrahedral geometry. The four identical C–Cl bond dipoles cancel by vector addition, giving μ = 0 (NCERT Class 11 Chemistry Chapter 4, page 112).

Why B is wrong: B: NH₃ is trigonal pyramidal with a lone pair reinforcing the bond dipoles. μ ≈ 1.47 D.

Why C is wrong: C: H₂O is bent (104.5°), so the two O–H bond dipoles do not cancel. μ ≈ 1.85 D.

Why D is wrong: D: CHCl₃ lacks the symmetric substitution of CCl₄ — one H replaces a Cl, breaking tetrahedral symmetry. μ ≠ 0.

Answer: A. 1 Debye = 3.336 × 10⁻³⁰ C·m. This conversion is directly stated in NCERT Class 11 Chemistry Chapter 4, page 111.

Why B is wrong: B uses 10⁻²⁰ instead of 10⁻³⁰ — a common power-of-ten confusion.

Why C is wrong: C is the elementary charge (e = 1.602 × 10⁻¹⁹ C), not the Debye conversion factor.

Why D is wrong: D is Avogadro's number scaled down — unrelated to dipole moment units.

Answer: B. BF₃ is trigonal planar (120° bond angles). The three identical B–F bond dipoles are symmetrically arranged, and their vector sum is zero (NCERT Class 11 Chemistry Chapter 4, page 111).

Why A is wrong: A: B–F bonds ARE polar — fluorine is highly electronegative. The individual bonds have significant dipole moments; it is the molecular symmetry that causes net cancellation.

Why C is wrong: C: Boron in BF₃ has zero lone pairs (it is electron-deficient). This is a common confusion with molecules like NH₃ where lone pairs contribute to the net dipole.

Why D is wrong: D: Fluorine is the MOST electronegative element. B–F bonds are decidedly polar.

Answer: B. H₂O has the highest dipole moment (~1.85 D) because oxygen is the most electronegative central atom, creating the largest charge separation. The bond angle in H₂O (104.5°) also contributes to a larger net vector compared to the narrower angles in H₂S and H₂Se (NCERT Class 11 Chemistry Chapter 4, pages 111–112 give H₂O 1.85 D and H₂S 0.95 D; H₂Se is beyond NCERT).

Why A is wrong: A: Selenium is less electronegative than oxygen or sulfur, so H₂Se has the smallest bond polarity and hence the lowest dipole moment in this series.

Why C is wrong: C: H₂S has a lower dipole moment than H₂O because sulfur is less electronegative than oxygen, resulting in less charge separation.

Why D is wrong: D: Electronegativity of the central atom differs significantly (O > S > Se), so the dipole moments are not equal.

Answer: C. In NH₃, the lone pair on N points in the same direction as the resultant of the three N–H bond dipoles (both away from N toward the lone pair side), reinforcing the net dipole. In NF₃, the bond dipoles point toward F (more electronegative), which is opposite to the lone-pair dipole direction. The partial cancellation drastically reduces μ (NCERT Class 11 Chemistry Chapter 4, page 112).

Why A is wrong: A: N–F bonds are highly polar (fluorine is the most electronegative element). The low net dipole is due to vector opposition with the lone pair, not nonpolarity.

Why B is wrong: B: Both NF₃ and NH₃ are trigonal pyramidal. NF₃ is NOT planar — nitrogen has one lone pair in both molecules.

Why D is wrong: D: Fluorine is far more electronegative than nitrogen and withdraws electrons, not donates them.

Answer: D. μ = q × d = 1.6 × 10⁻¹⁹ × 1.0 × 10⁻¹⁰ = 1.6 × 10⁻²⁹ C·m. Converting: 1.6 × 10⁻²⁹ / 3.336 × 10⁻³⁰ ≈ 4.80 D (NCERT Class 11 Chemistry Chapter 4, page 111).

Why A is wrong: A: 48.0 D is ten times the correct value, from a power-of-ten slip in the conversion factor (dividing by 3.336 × 10⁻³¹ instead of 3.336 × 10⁻³⁰).

Why B is wrong: B: 1.60 D takes the number in front of μ = 1.6 × 10⁻²⁹ C·m as if it were already in debye, skipping the conversion (1 D = 3.336 × 10⁻³⁰ C·m).

Why C is wrong: C: This results from a power-of-ten error during the C·m to Debye conversion (dividing by 3.336 × 10⁻²⁹ instead of 3.336 × 10⁻³⁰).

Answer: C. CO₂ is linear and symmetric — the two C=O dipoles cancel (μ = 0). SO₂ is bent (~119°), so the two S=O dipoles do not cancel (μ ≈ 1.63 D). The key distinction is geometry: linear symmetric vs bent (NCERT Class 11 Chemistry Chapter 4, page 112 lists CO₂ as linear with zero dipole moment; SO₂ is beyond NCERT).

Why A is wrong: A: This is reversed. CCl₄ is symmetric tetrahedral (μ = 0). CHCl₃ has asymmetric substitution (μ ≠ 0).

Why B is wrong: B: This is reversed. NH₃ is trigonal pyramidal with μ = 1.47 D (non-zero). BF₃ is trigonal planar with μ = 0 (symmetric cancellation).

Why D is wrong: D: CH₄ is correctly zero (symmetric tetrahedral), but CO₂ is also zero (linear symmetric). Neither molecule of this pair has a non-zero dipole moment.