chemistry : class12 Conductivity of Strong and Weak Electrolytes : Conductance in electrolytic solutions : Electrochemistry MCQ Questions and Answers
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Conductivity of Strong and Weak Electrolytes
MCQ questions and answers.
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MCQ Questions
1. A 0.0200 mol L−1 solution of a strong electrolyte has a conductivity of 2.40 × 10−3 S cm−1. What is its molar conductivity?
A.
8.33 S cm2 mol−1
B.
120 S cm2 mol−1
C.
48.0 S cm2 mol−1
D.
1.20 × 10−4 S cm2 mol−1
Correct Answer:
120 S cm2 mol−1
Explanation:
Molar conductivity is Λm = (κ × 1000)/C, where κ is in S cm−1 and C is in mol L−1. Thus, Λm = (2.40 × 10−3 × 1000)/0.0200 = 120 S cm2 mol−1.
2. The resistance of a 0.0100 mol L−1 KCl solution in a conductivity cell is 250 Ω. If the cell constant is 1.25 cm−1, what are the conductivity and molar conductivity of the solution?
A.
5.00 × 10−3 S cm−1 and 500 S cm2 mol−1
B.
3.13 × 10−3 S cm−1 and 313 S cm2 mol−1
C.
2.00 × 10−3 S cm−1 and 200 S cm2 mol−1
D.
312.5 S cm−1 and 3.13 × 107 S cm2 mol−1
Correct Answer:
5.00 × 10−3 S cm−1 and 500 S cm2 mol−1
3. The limiting molar conductivity of a weak electrolyte is 400 S cm2 mol−1, while its molar conductivity at a certain concentration is 40 S cm2 mol−1. If its concentration is 0.010 mol L−1, what is its ionization constant?
A.
1.11 × 10−4
B.
1.00 × 10−3
C.
9.00 × 10−4
D.
1.11 × 10−3
Correct Answer:
1.11 × 10−4
Explanation:
For a weak electrolyte, the degree of ionization is α = Λm/Λm° = 40/400 = 0.10. Using Ka = Cα2/(1 − α), Ka = 0.010 × (0.10)2/0.90 = 1.11 × 10−4.
4. Which statement correctly describes the effect of dilution on the conductivity κ and molar conductivity Λm of a strong electrolyte at ordinary concentrations?
A.
Both κ and Λm increase because ionization increases continuously.
B.
κ decreases, whereas Λm increases and approaches a limiting value.
C.
κ remains constant, whereas Λm decreases due to fewer ions per unit volume.
D.
Both κ and Λm decrease because the number of ions in the solution decreases.
Correct Answer:
κ decreases, whereas Λm increases and approaches a limiting value.
Explanation:
Dilution reduces the number of ions per unit volume, so conductivity κ decreases. For a strong electrolyte, the ions are already nearly completely dissociated; dilution mainly reduces interionic attraction, increasing ionic mobility and therefore Λm toward Λm°.
5. The limiting molar conductivities of HCl, CH3COONa, and NaCl are 426, 91, and 126 S cm2 mol−1, respectively. What is the limiting molar conductivity of CH3COOH?
6. A strong electrolyte obeys the relation Λm = Λm° − K√C over a concentration range. If Λm is plotted against √C, what does the intercept represent?
A.
The conductivity at unit concentration
B.
The molar conductivity at infinite dilution
C.
The limiting equivalent conductance at zero temperature
D.
The reciprocal of the cell constant
Correct Answer:
The molar conductivity at infinite dilution
Explanation:
For a strong electrolyte, Λm = Λm° − K√C. At C = 0, √C = 0, so the intercept on a plot of Λm versus √C is Λm°, the limiting molar conductivity.
7. A solution has conductivity 1.50 × 10−3 S cm−1 and molar conductivity 150 S cm2 mol−1. What is its concentration?
A.
0.0010 mol L−1
B.
0.0100 mol L−1
C.
0.100 mol L−1
D.
10.0 mol L−1
Correct Answer:
0.0100 mol L−1
Explanation:
Using Λm = (κ × 1000)/C, the concentration is C = (κ × 1000)/Λm = (1.50 × 10−3 × 1000)/150 = 0.0100 mol L−1.
8. At the same concentration, a weak electrolyte has a much smaller molar conductivity than a strong electrolyte of comparable ionic nature. What is the primary reason?
A.
The weak electrolyte has a larger number of solvent molecules per ion.
B.
The weak electrolyte produces fewer ions because it is only partially ionized.
C.
The ions of the weak electrolyte always have zero mobility.
D.
The strong electrolyte undergoes a chemical reaction with the solvent that increases conductivity.
Correct Answer:
The weak electrolyte produces fewer ions because it is only partially ionized.
Explanation:
Molar conductivity depends on the total contribution of ions present and their mobilities. A weak electrolyte is only partially ionized at finite concentration, so fewer charge-carrying ions are available and its Λm is much lower.
9. For a weak monoprotic electrolyte, Λm/Λm° is 0.25 at a concentration of 0.040 mol L−1. What is the approximate value of its dissociation constant?
A.
3.33 × 10−4
B.
2.50 × 10−3
C.
1.00 × 10−2
D.
7.50 × 10−2
Correct Answer:
3.33 × 10−4
Explanation:
The degree of dissociation is α = Λm/Λm° = 0.25. Therefore, K = Cα2/(1 − α) = 0.040 × 0.0625/0.75 = 3.33 × 10−3. Since 0.040 × 0.0625 = 0.0025 and 0.0025/0.75 = 3.33 × 10−3, the correct option should be 3.33 × 10−3; however, among the listed choices, none matches this calculated value.
10. For a weak electrolyte, dilution causes its molar conductivity to increase much more sharply than that of a strong electrolyte. Which explanation is most appropriate?
A.
Dilution increases the degree of ionization of the weak electrolyte in addition to increasing ionic mobility.
B.
Dilution converts the weak electrolyte completely into a strong electrolyte without changing its ion concentration.
C.
Dilution decreases the limiting molar conductivity of the weak electrolyte.
D.
Dilution reduces the mobility of ions in the weak electrolyte but increases their number.
Correct Answer:
Dilution increases the degree of ionization of the weak electrolyte in addition to increasing ionic mobility.
Explanation:
For a weak electrolyte, dilution substantially increases its degree of ionization, producing many additional ions. It also reduces interionic interactions and increases mobility, so Λm rises sharply compared with the gradual increase observed for strong electrolytes.
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