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Electrolytic/metallic conduction and molar conductivity: NEET MCQs with answers

90 questions on this topic of Redox Reactions and Electrochemistry (NEET 2027 syllabus). Here are 12; tap one to see its answer and explanation.

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easy

1. The limiting molar conductivity of Ca²⁺ and Cl⁻ ions are 119.0 S cm² mol⁻¹ and 76.3 S cm² mol⁻¹ respectively. The limiting molar conductivity of CaCl₂ is:

  1. 195.3 S cm² mol⁻¹
  2. 271.6 S cm² mol⁻¹
  3. 238.0 S cm² mol⁻¹
  4. 152.6 S cm² mol⁻¹
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Answer: (B) 271.6 S cm² mol⁻¹

According to Kohlrausch's Law of independent migration of ions, Λm°(CaCl₂) = λ°(Ca²⁺) + 2λ°(Cl⁻). Thus, Λm°(CaCl₂) = 119.0 + 2(76.3) = 119.0 + 152.6 = 271.6 S cm² mol⁻¹.

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2. Consider the following statements regarding electrical conductance: Statement I: Electronic conduction decreases with increase in temperature due to increased lattice vibrations. Statement II: Electrolytic conduction increases with increase in temperature due to decrease in solvent viscosity and increase in ionic mobilities. In the light of the above statements, choose the correct answer:

  1. Both Statement I and Statement II are correct
  2. Statement I is correct but Statement II is incorrect
  3. Statement I is incorrect but Statement II is correct
  4. Both Statement I and Statement II are incorrect
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Answer: (A) Both Statement I and Statement II are correct

Electronic conduction in metals involves movement of electrons; as temperature increases, metal kernels vibrate more vigorously, offering greater resistance. In electrolytic solutions, higher temperature weakens interionic attractions and reduces solvent viscosity, thus increasing ionic mobility and conductance. Both statements are correct.

hard

3. A saturated solution of a sparingly soluble salt AB₂ has a specific conductance (conductivity) of 4.0 × 10⁻⁵ S cm⁻¹ (after subtracting the conductivity of water). If the limiting molar conductivity of AB₂ is 200 S cm² mol⁻¹, what is the solubility product (Ksp) of AB₂ at this temperature?

  1. 4.0 × 10⁻⁸ mol³ L⁻³
  2. 1.6 × 10⁻¹¹ mol³ L⁻³
  3. 3.2 × 10⁻¹¹ mol³ L⁻³
  4. 1.08 × 10⁻¹⁰ mol³ L⁻³
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Answer: (C) 3.2 × 10⁻¹¹ mol³ L⁻³

Solubility s = (1000 × κ) / Λm° = (1000 × 4.0 × 10⁻⁵) / 200 = 0.04 / 200 = 2.0 × 10⁻⁴ mol L⁻¹. For AB₂ ⇌ A²⁺ + 2B⁻, Ksp = [A²⁺][B⁻]² = s × (2s)² = 4s³ = 4 × (2.0 × 10⁻⁴)³ = 4 × 8.0 × 10⁻¹² = 3.2 × 10⁻¹¹ mol³ L⁻³.

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4. Consider the following statements: Statement I: The molar conductivity of an electrolytic solution always increases on dilution for both strong and weak electrolytes. Statement II: For strong electrolytes, the increase in molar conductivity on dilution is mainly due to an increase in the number of ions, whereas for weak electrolytes, it is due to an increase in ionic mobility.

  1. Statement I is true, Statement II is false.
  2. Statement I is false, Statement II is true.
  3. Both Statement I and Statement II are true.
  4. Both Statement I and Statement II are false.
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Answer: (A) Statement I is true, Statement II is false.

Statement I is true: molar conductivity increases upon dilution for both strong and weak electrolytes because the total volume V containing one mole of electrolyte increases more than κ decreases (Λm = κ × V). Statement II is false: for strong electrolytes, the number of ions is already completely dissociated, so the increase is due to decreased interionic attractions (increased ionic mobility); for weak electrolytes, the sharp increase is primarily due to an increase in the degree of dissociation (number of ions).

easy

5. What is the unit of cell constant (G*) of a conductivity cell in CGS and SI units respectively?

  1. cm and m
  2. cm⁻¹ and m⁻¹
  3. S cm⁻¹ and S m⁻¹
  4. Ω cm and Ω m
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Answer: (B) cm⁻¹ and m⁻¹

Cell constant is defined as G* = l / A, where l is the distance between electrodes and A is the cross-sectional area. In CGS units it is expressed as cm/cm² = cm⁻¹, and in SI units as m/m² = m⁻¹.

hard

6. A conductivity cell filled with 0.05 M solution of a 1:1 electrolyte has a resistance of 100 Ω. The electrodes are 1.0 cm apart and each has a cross-sectional area of 2.0 cm². What is the molar conductivity (Λm) of this electrolytic solution in S m² mol⁻¹?

  1. 1.0 × 10⁻² S m² mol⁻¹
  2. 1.0 × 10⁻¹ S m² mol⁻¹
  3. 1.0 × 10² S m² mol⁻¹
  4. 5.0 × 10⁻³ S m² mol⁻¹
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Answer: (A) 1.0 × 10⁻² S m² mol⁻¹

Cell constant G* = l / A = 1.0 cm / 2.0 cm² = 0.5 cm⁻¹ = 50 m⁻¹. Conductivity κ = G* / R = 50 m⁻¹ / 100 Ω = 0.5 S m⁻¹. Concentration c = 0.05 mol L⁻¹ = 0.05 × 10³ mol m⁻³ = 50 mol m⁻³. In SI units, Λm = κ / c = 0.5 S m⁻¹ / 50 mol m⁻³ = 0.01 S m² mol⁻¹ = 1.0 × 10⁻² S m² mol⁻¹.

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7. For three 1-1 electrolytes NaCl, KCl, and LiCl, which of the following represents the correct order of molar conductivity at infinite dilution (Λm°) in aqueous solution at 298 K?

  1. NaCl > KCl > LiCl
  2. LiCl > NaCl > KCl
  3. KCl > NaCl > LiCl
  4. KCl > LiCl > NaCl
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Answer: (C) KCl > NaCl > LiCl

All three electrolytes share the same anion (Cl⁻). Down the group 1 cations, the ionic radius increases (Li⁺ < Na⁺ < K⁺), meaning charge density decreases and hydration decreases. Consequently, the hydrated ionic radius decreases (Li⁺(aq) > Na⁺(aq) > K⁺(aq)), and ionic mobility increases: K⁺ > Na⁺ > Li⁺. Therefore, Λm°(KCl) > Λm°(NaCl) > Λm°(LiCl).

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8. Why does molar conductivity (Λm) of a weak electrolyte increase steeply upon dilution near zero concentration?

  1. Because the interionic attraction completely disappears while the number of ions remains constant
  2. Because degree of dissociation increases sharply and approaches unity as concentration approaches zero
  3. Because the solvent completely evaporates near infinite dilution
  4. Because weak electrolytes completely dissociate at all concentrations
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Answer: (B) Because degree of dissociation increases sharply and approaches unity as concentration approaches zero

For a weak electrolyte (e.g., CH₃COOH), the degree of dissociation (α) is very small at moderate concentrations. According to Ostwald's dilution law, α increases steeply as dilution approaches infinite dilution (c → 0), causing a sharp increase in the number of free ions per mole of solute, and hence a steep rise in Λm.

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9. A conductivity cell filled with 0.1 M KCl solution has a resistance of 250 Ω. When the same cell is filled with a 0.02 M solution of an electrolyte XY, the measured resistance is 1000 Ω. If the conductivity of 0.1 M KCl is 1.29 S m⁻¹, what is the molar conductivity (Λm) of the XY solution?

  1. 1.61 × 10⁻⁴ S m² mol⁻¹
  2. 3.225 × 10⁻³ S m² mol⁻¹
  3. 1.61 × 10⁻² S m² mol⁻¹
  4. 6.45 × 10⁻³ S m² mol⁻¹
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Answer: (C) 1.61 × 10⁻² S m² mol⁻¹

Cell constant G* = R₁ × κ₁ = 250 Ω × 1.29 S m⁻¹ = 322.5 m⁻¹. Conductivity of the XY solution is κ₂ = G* / R₂ = 322.5 / 1000 = 0.3225 S m⁻¹. Concentration c = 0.02 mol L⁻¹ = 0.02 × 1000 mol m⁻³ = 20 mol m⁻³. Therefore, Λm = κ₂ / c = 0.3225 / 20 = 0.016125 S m² mol⁻¹ = 1.61 × 10⁻² S m² mol⁻¹.

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10. The electrical resistance of a column of 0.05 mol L⁻¹ NaOH solution of diameter 1 cm and length 50 cm is 5.5 × 10³ Ω. Its conductivity (κ) in S cm⁻¹ is approximately: (Take π = 3.14)

  1. 5.80 × 10⁻³ S cm⁻¹
  2. 2.32 × 10⁻² S cm⁻¹
  3. 0.87 × 10⁻² S cm⁻¹
  4. 1.16 × 10⁻² S cm⁻¹
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Answer: (D) 1.16 × 10⁻² S cm⁻¹

Area of cross-section A = πr² = 3.14 × (0.5 cm)² = 0.785 cm². Cell constant G* = l / A = 50 cm / 0.785 cm² = 63.69 cm⁻¹. Conductivity κ = G* / R = 63.69 cm⁻¹ / (5.5 × 10³ Ω) ≈ 0.01158 S cm⁻¹ = 1.16 × 10⁻² S cm⁻¹.

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11. With increase in temperature, the conductivity of:

  1. Both metals and electrolytes increases
  2. Both decreases
  3. Metals increases, while that of electrolytes decreases
  4. Metals decreases, while that of electrolytes increases
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Answer: (D) Metals decreases, while that of electrolytes increases

Metallic conduction decreases with temperature because of increased lattice vibrations. Electrolytic conduction increases because ions move faster and viscosity decreases.

NCERT Class 11

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12. Equal volumes of 0.1 M HCl and 0.1 M NaCl are compared at 298 K. The conductivity of the HCl solution is found to be significantly higher than that of the NaCl solution. The primary reason for this observation is:

  1. HCl is a stronger electrolyte than NaCl and dissociates to a greater extent.
  2. H⁺ ions possess an exceptionally high molar ionic conductivity due to the Grotthuss proton-jumping mechanism in water.
  3. Cl⁻ ions have a higher ionic mobility in an acidic medium than in a neutral medium.
  4. Na⁺ ions are smaller than H⁺ ions and experience greater frictional drag in water.
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Answer: (B) H⁺ ions possess an exceptionally high molar ionic conductivity due to the Grotthuss proton-jumping mechanism in water.

Both HCl and NaCl are strong electrolytes and are fully ionised in 0.1 M aqueous solutions. However, H⁺ (as H₃O⁺) conducts electricity extraordinarily fast in aqueous solution via proton transfer across hydrogen-bonded networks of water molecules (Grotthuss mechanism), making its ionic conductivity (λ° ≈ 349.6 S cm² mol⁻¹) far greater than that of Na⁺ (λ° ≈ 50.1 S cm² mol⁻¹).

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