chemistry : class12 Alternating Current Conductometry : Conductance in electrolytic solutions : Electrochemistry MCQ Questions and Answers
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Alternating Current Conductometry
MCQ questions and answers.
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MCQ Questions
1. In alternating-current conductometry, the use of an alternating voltage instead of direct voltage primarily minimizes which effect at the electrodes?
A.
Variation of the cell constant with temperature
B.
Electrode polarization caused by charge accumulation and electrolysis
C.
Decrease in the ionic mobility of the electrolyte
D.
Formation of a concentration gradient in the bulk solution
Correct Answer:
Electrode polarization caused by charge accumulation and electrolysis
Explanation:
Alternating current periodically reverses the direction of ion migration, greatly reducing sustained charge accumulation and electrode polarization. This permits the conductance of the solution to be measured more accurately.
2. A conductivity cell has a cell constant of 1.50 cm⁻¹. When filled with a solution, its measured conductance by an AC conductometric bridge is 2.00 × 10⁻³ S. What is the conductivity of the solution?
A.
1.33 × 10⁻³ S cm⁻¹
B.
3.00 × 10⁻³ S cm⁻¹
C.
7.50 × 10⁻⁴ S cm⁻¹
D.
3.50 × 10⁻³ S cm⁻¹
Correct Answer:
3.00 × 10⁻³ S cm⁻¹
Explanation:
Conductivity is calculated from κ = G × cell constant. Therefore, κ = (2.00 × 10⁻³ S)(1.50 cm⁻¹) = 3.00 × 10⁻³ S cm⁻¹.
3. In an AC conductivity measurement, the solution resistance is represented by an equivalent conductance G in parallel with the electrode double-layer capacitance. If the frequency is increased sufficiently, what is the principal effect on the measured response?
A.
The capacitive reactance increases and electrode polarization becomes stronger
B.
The capacitive reactance decreases, allowing the solution conductance to be measured with reduced polarization error
C.
The solution conductance becomes zero because ions cannot follow the field
D.
The cell constant changes solely because of the applied frequency
Correct Answer:
The capacitive reactance decreases, allowing the solution conductance to be measured with reduced polarization error
Explanation:
The capacitive reactance is Xc = 1/(2πfC), so it decreases as frequency increases. The alternating-current method uses a suitable frequency to reduce electrode polarization and obtain a response closer to the true solution conductance.
4. Why is a Wheatstone-type bridge used with an AC source in conductometric measurements instead of simply measuring current and voltage with a DC source?
A.
The bridge eliminates the need for a conductivity cell
B.
The bridge permits comparison of the unknown cell impedance with known resistances while avoiding direct-current electrode polarization
C.
The bridge converts a strong electrolyte into a weak electrolyte
D.
The bridge makes conductivity independent of temperature
Correct Answer:
The bridge permits comparison of the unknown cell impedance with known resistances while avoiding direct-current electrode polarization
Explanation:
An AC bridge determines the electrical response of the conductivity cell by balancing it against known components. Alternating excitation also avoids the persistent electrode polarization associated with direct current.
5. During AC conductometry, the conductivity of an electrolyte solution is determined from its measured conductance and the cell constant. Which change would alter the measured conductance but not the cell constant?
A.
Replacing the conductivity cell with one having a different electrode separation
B.
Changing the concentration of the electrolyte in the same cell
C.
Changing the electrode area while retaining the same solution
D.
Using a cell calibrated with a different standard solution
Correct Answer:
Changing the concentration of the electrolyte in the same cell
Explanation:
The cell constant depends on the geometry of the cell, approximately l/A, and remains fixed for a given cell. Changing electrolyte concentration changes the solution conductivity and hence the measured conductance, but not the cell constant.
6. A conductivity cell is connected to an AC source. At the selected frequency, the bridge balance gives a resistance of 250 Ω for the cell, and the cell constant is 1.25 cm⁻¹. What is the conductivity of the solution?
A.
5.00 × 10⁻³ S cm⁻¹
B.
3.13 × 10⁻³ S cm⁻¹
C.
2.00 × 10⁻³ S cm⁻¹
D.
1.25 × 10⁻² S cm⁻¹
Correct Answer:
5.00 × 10⁻³ S cm⁻¹
Explanation:
The conductance is G = 1/R = 1/250 = 4.00 × 10⁻³ S. Hence κ = G × cell constant = (4.00 × 10⁻³)(1.25) = 5.00 × 10⁻³ S cm⁻¹.
7. In an AC conductometric titration, the conductance does not always vary linearly with the volume of titrant added even when the reaction is complete. Which factor is most directly responsible for this deviation?
A.
Dilution of the solution as titrant is added
B.
The cell constant becoming zero after equivalence
C.
Alternating current changing the stoichiometric coefficients
D.
The electrodes becoming chemically inert only before equivalence
Correct Answer:
Dilution of the solution as titrant is added
Explanation:
Addition of titrant increases the total volume, causing dilution. This changes ion concentrations and therefore conductance, so dilution correction or suitable extrapolation is needed when interpreting AC conductometric titration curves.
8. A conductivity cell is calibrated using a standard KCl solution of known conductivity. Why is calibration necessary before measuring an unknown solution by AC conductometry?
A.
To determine the frequency at which KCl undergoes electrolysis
B.
To determine the cell constant, which accounts for the geometrical arrangement of the electrodes
C.
To remove all ionic impurities from the unknown solution
D.
To make the conductivity of the unknown equal to that of KCl
Correct Answer:
To determine the cell constant, which accounts for the geometrical arrangement of the electrodes
Explanation:
The measured conductance depends on both the solution conductivity and cell geometry. A standard solution with known conductivity allows calculation of the cell constant, which is then used to obtain the conductivity of the unknown.
9. For an ideal resistor representing the bulk solution in an AC conductivity cell, which phase relationship is expected between the alternating voltage across the solution and the current through it?
A.
The current leads the voltage by 90°
B.
The current lags the voltage by 90°
C.
The current and voltage are in phase
D.
The current and voltage always differ by 180°
Correct Answer:
The current and voltage are in phase
Explanation:
The bulk electrolytic conductance is represented ideally by a resistance. In a purely resistive element, alternating current and voltage are in phase; phase shifts mainly arise from capacitive effects such as electrode polarization.
10. Two conductivity cells contain the same electrolyte solution and are operated at the same temperature and AC frequency. Cell X has electrodes separated by a greater distance than cell Y, while their electrode areas are equal. Which statement is correct?
A.
Cell X has the smaller cell constant and the greater conductance
B.
Cell X has the greater cell constant and the smaller conductance
C.
Both cells have the same cell constant but different conductivity
D.
Cell Y has the greater cell constant because its electrodes are closer
Correct Answer:
Cell X has the greater cell constant and the smaller conductance
Explanation:
The cell constant is l/A. With equal electrode areas, increasing the separation l increases the cell constant. Since G = κA/l, the same solution gives lower conductance in cell X.
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