Q.(a) Define specific conductance and molar conductance. What is the effect of dilution on specific and molar conductances of an electrolyte ? [2+2=4]

Showing the 12 most recent of 27 on this concept.

1. CBSE 2026Set A1 markMCQ
   Q.On increasing dilution, the specific conductance of an electrolyte

   (a) increases

   (b) decreases

   (c) remains constant

   (d) none of these

   ›Reveal solutionSolution

   Specific conductance (conductance per unit volume) falls on dilution because the number of current-carrying ions per unit volume decreases.

   Specific conductance (κ) is the conductance of a solution held between electrodes 1 cm apart with 1 cm² area, i.e. conductance of unit volume. On dilution the number of ions per unit volume decreases, …

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2. CBSE 2026Set A1 markMCQ
   Q.The number of ions in aqueous solution of [Co(NH3)5Cl]Cl2 is

   (a) 3

   (b) 4

   (c) 2

   (d) 6

   ›Reveal solutionSolution

   Only the ions outside the coordination sphere are free; [Co(NH3)5Cl]Cl2 gives one complex cation plus two chloride ions = 3 ions.

   In a coordination compound, only the counter ions outside the square brackets dissociate in water; the ligands inside the coordination sphere stay bound to the metal. Here one Cl and five NH3 are coordinated to cobalt, and two Cl are counter ions:

   [Co(NH3)5Cl]Cl2 -> [Co(NH3)5Cl]2+ + 2 Cl-

   …

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3. CBSE 2026Set ANNUAL1 markMCQ
   Q.The unit of cell constant is:

   (a) Ohm^-1 cm^2

   (b) cm^-1

   (c) Ohm^-1 cm^-1

   (d) Ohm^-1 cm^2/ g eq

   ›Reveal solutionSolution

   Cell constant $G^{*}=l/A$ has the unit of reciprocal length, i.e. cm^-1.

   The cell constant of a conductivity cell is defined as the ratio of the distance between the two electrodes (l) to the area of cross-section of the electrodes (A): $G^{*} = \dfrac{l}{A}$. Since l has units of cm and A has units of cm^2, the cell constant has units of $\dfrac{cm}{cm^2} = cm^{-1}$.

   …

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4. CBSE 2026Set ANNUAL1 markMCQ
   Q.The unit of specific conductivity is:

   (a) ohm⁻¹

   (b) ohm⁻¹ cm⁻¹

   (c) ohm cm

   (d) ohm cm⁻¹

   ›Reveal solutionSolution

   Specific conductance ($\kappa$) is measured in ohm⁻¹ cm⁻¹ (S cm⁻¹).

   Specific conductivity ($\kappa$), also called conductivity, is the conductance of a 1 cm cube of a solution of an electrolyte. Conductance ($G$) is the reciprocal of resistance and is measured in ohm⁻¹ (siemens, S). Since $\kappa = G \times (l/A)$, where $l/A$ (t …

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5. CBSE 2025Set 56/5/11 markMCQ
   Q.Two statements are given — one labelled as Assertion (A) and the other labelled as Reason (R). Select the correct answer from the codes (A), (B), (C) and (D) as given below. (A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A). (B) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). (C) Assertion (A) is true, but Reason (R) is false. (D) Assertion (A) is false, but Reason (R) is true. Assertion (A) : For measuring resistance of an ionic solution an AC source is used. Reason (R) : Concentration of ionic solution will change if DC source is used.

   ›Reveal solutionSolution

   AC is used to measure ionic solution resistance because DC causes electrolysis, which changes the solution's composition and hence its conductance; both statements are true and the reason correctly explains the assertion.

   When we measure the resistance (or conductance) of an ionic solution, we're essentially probing how easily ions can carry current through the liquid. The choice between AC and DC isn't arbitrary—it stems from what happens at the electrode-solution interface.

   Why DC causes problems

   In an ionic solution, current flows via the movement of ions: cations migrate toward the cathode, anions toward the anode. With a DC source, these ions don't just move—they undergo redox reactions at the electrodes. For instance, in a $\text{NaCl}$ solution, $\text{Cl}^-$ ions get oxidized at the anode ($2\text{Cl}^- \to \text{Cl}_2 + 2e^-$) and $\text{H}^+$ from water gets reduced at the cathode ($2\text{H}^+ + 2e^- \to \text{H}_2$). This is electrolysis.

   The consequence? The concentration of ions in the solution changes continuously. As ions are consumed or new species are produced, the conductance of the solution drifts. You're no longer measuring the property of the original solution—you're measuring a changing system. The reading becomes unreliable and time-dependent.

   Why AC solves this

   An alternating current reverses direction many times per second (typically at 1000 Hz or so in conductivity bridges). In one half-cycle, a tiny bit of electrolysis might begin, but in the next half-cycle the current reverses and the reaction is essentially undone. The net chemical change over many cycles is negligible. The solution composition remains stable, and the resistance measurement reflects the true, steady-state property of the ionic solution. …

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6. CBSE 2025Set D1 markMCQ
   Q.The unit of specific conductance is

   (a) ohm cm^-1

   (b) ohm cm^-2

   (c) ohm^-1 cm^-1

   (d) ohm^-1 cm^-2

   ›Reveal solutionSolution

   Specific conductance = 1/(specific resistance), so its unit is ohm^-1 cm^-1 (S cm^-1).

   Specific conductance (conductivity), kappa, is the reciprocal of specific resistance (resistivity), rho:

   kappa = 1/rho

   Specific resistance has the unit ohm cm, so its reciprocal has the unit:

   …

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7. CBSE 2025Set A1 mark
   Q.Write the value of conductivity of superconductor.

   ›Reveal solutionSolution

   Since conductivity is the reciprocal of resistivity, and a superconductor's resistivity drops to exactly zero, its conductivity becomes infinite.

   Certain materials, when cooled below a characteristic critical temperature, lose all electrical resistance completely — this state is called superconductivity, and such materials are superconductors. Electrical conductivity ($\kappa$) and resistivity ($\rho$) are reciprocals of each other: $\kappa = 1/\rho$. Because a superconductor's res …

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8. CBSE 2025Set ANNUAL1 markMCQ
   Q.SI unit of resistivity (specific resistance) is -

   (a) Ω

   (b) Ω^-1

   (c) Ωm

   (d) Ωm^-1

   ›Reveal solutionSolution

   Resistivity (specific resistance) has SI unit ohm-metre (Ωm).

   Resistance of a conductor is related to its resistivity by:

   R = rho x (l/A)

   where l is length (m) and A is cross-sectional area (m^2). Rearranging:

   rho = R x A / l

   Units: rho = (ohm) x (m^2) / (m) = ohm x m = Ωm

   …

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9. CBSE 2024Set D1 markMCQ
   Q.Which of the following has the highest molar electrical conductance in aqueous solution?

   (a) [Pt(NH3)6]Cl4

   (b) [Pt(NH3)5Cl]Cl3

   (c) [Pt(NH3)4Cl2]Cl2

   (d) [Pt(NH3)3Cl3]Cl

   ›Reveal solutionSolution

   Molar conductance rises with the number of ions produced on dissociation. [Pt(NH3)6]Cl4 gives 5 ions, the most of the options, so it conducts best.

   Count the ions each complex furnishes in water (only the counter-ions outside the coordination sphere ionise):

   • [Pt(NH3)6]Cl4 -> [Pt(NH3)6]4+ + 4 Cl- => 5 ions
   • [Pt(NH3)5Cl]Cl3 -> [Pt(NH3)5Cl]3+ + 3 Cl- => 4 ions
   • [Pt(NH3)4Cl2]Cl2 -> [Pt(NH3)4Cl2]2+ + 2 Cl- => 3 ions …

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10. CBSE 2024Set D1 markMCQ
    Q.The cell constant of a conductivity cell is

    (a) l/A

    (b) A/l

    (c) l.A

    (d) R/A

    ›Reveal solutionSolution

    Cell constant = l/A (distance between electrodes ÷ electrode area), unit cm^-1.

    Conductance G of a solution in a conductivity cell is G = kappa (A/l), where kappa is conductivity, A is the electrode area and l is the distance between the electrodes.

    Rearranging, kappa = G (l/A). The geometric factor (l/A) is called the CELL CONSTANT because it depends only on the fixed geometry of the cell.

    …

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11. CBSE 2024Set B1 markMCQ
    Q.The unit of cell constant is

    (a) ohm cm

    (b) cm^-1

    (c) cm

    (d) ohm^-1 cm^-1

    ›Reveal solutionSolution

    Cell constant G* = l/A (distance between electrodes divided by their area), so its unit works out to cm^-1.

    For a conductivity cell, the cell constant is defined as:

    $G^{*} = \frac{l}{A}$

    where l is the distance between the two electrodes (in cm) and A is the area of cross-section of the electrodes (in cm^2).

    Units: cm / cm^2 = cm^-1.

    …

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12. CBSE 2024Set ANNUAL1 markMCQ
    Q.SI unit of conductivity is -

    (a) S

    (b) Ω

    (c) S cm^-1

    (d) S m^-1

    ›Reveal solutionSolution

    Conductivity kappa = conductance x (l/A); in SI units this comes out as siemens per metre (S m^-1).

    Conductance G has the unit siemens (S), where S = ohm^-1 = A/V.

    Conductivity kappa is defined as kappa = G x (l/A), where l is the length between electrodes (in metres) and A is the cross-sectional area (in square metres).

    So the unit of kappa = S x (m/m^2) = S m^-1.

    Option (a) S is just the unit of conductance, not conductivity. …

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