Rahul set-up an experiment to find resistance of aqueous KCl solution for different concentrations at 298 K using a conductivity cell connected to a Wheatstone bridge. He fed the Wheatstone bridge with a.c. power in the audio frequency range 550 to 5000 cycles per second. Once the resistance was calculated from null point he also calculated the conductivity K and molar conductivity _m and recorded his readings in tabular form. S.No. Conc.(M) k S cm⁻¹ _m S cm² mol⁻¹ --- --- --- --- 1. 1.00 111.3 × 10^-3 111.3 2. 0.10 12.9 × 10^-3 129.0 3. 0.01 1.41 × 10^-3 141.0 (a) Why does conductivity decrease with dilution ? (b) If _m^o of KCl is 150.0 S cm² mol⁻¹, calculate the degree of dissociation of 0.01 M KCl. (c) If Rahul had used HCl instead to KCl then would you expect the _m values to be more or less than those per KCl for a given concentration. Justify. (c) Amit, a classmate of Rahul repeated the same experiment with CH_3COOH solution instead of KCl solution. Give one point that would be similar and one that would be different in his observations as compared to Rahul.

ChemistryCBSE Class 12 · Board exam 2023, set 56/5/1 · 4 marks · Case-based

CBSE Class 12 Chemistry 2023: question 32

Set in the CBSE Class 12 board exam of 2023, paper 56/5/1, section D, question 32, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q32 · 4 marks · Case-basedCompetency-basedElectrochemistry

Rahul set-up an experiment to find resistance of aqueous KCl solution for different concentrations at 298 K using a conductivity cell connected to a Wheatstone bridge. He fed the Wheatstone bridge with a.c. power in the audio frequency range 550 to 5000 cycles per second. Once the resistance was calculated from null point he also calculated the conductivity K and molar conductivity $\land_{m}$ and recorded his readings in tabular form.

S.No.	Conc.(M)	k S cm⁻¹	$\land_{m}$ S cm² mol⁻¹
1.	1.00	$111.3 \times 1 0^{- 3}$	111.3
2.	0.10	$12.9 \times 1 0^{- 3}$	129.0
3.	0.01	$1.41 \times 1 0^{- 3}$	141.0

(a)
Why does conductivity decrease with dilution ?
1 mark
(b)
If $\land_{m}^{o}$ of KCl is 150.0 S cm² mol⁻¹, calculate the degree of dissociation of 0.01 M KCl.
1 mark
(c)
If Rahul had used HCl instead to KCl then would you expect the $\land_{m}$ values to be more or less than those per KCl for a given concentration. Justify.
2 marks
OR
(c)
Amit, a classmate of Rahul repeated the same experiment with $C H_{3} COOH$ solution instead of KCl solution. Give one point that would be similar and one that would be different in his observations as compared to Rahul.
2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 56/5/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Conductivity refers to a fixed unit volume of solution — what happens to the ions in it when you dilute?

Hint 2

Degree of dissociation = Λm at that concentration ÷ limiting molar conductivity.

Hint 3

Compare the ionic mobilities of H⁺ and K⁺; for acetic acid, recall how a weak electrolyte behaves on dilution.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) Conductivity is the conductance of 1 cm³ of solution; on dilution the number of ions per unit volume decreases, so conductivity decreases.
1 mark
(b) $α = \frac{\land _{m}^{c}}{\land _{m}^{o}} = \frac{141.0}{150.0} = 0.94$
1 mark
(c) $\land_{m}$ values for HCl would be more than those for KCl at the same concentration.
1 mark
Justification: $H^{+}$ has a much higher ionic mobility (limiting molar conductivity ≈ 349.6 S cm² mol⁻¹) than $K^{+}$ (≈ 73.5 S cm² mol⁻¹), because it moves by proton-hopping through H-bonded water molecules; the anion ( $C l^{-}$ ) is the same.
1 mark
OR (c) [alternative, 2 marks]
Similarity: for both KCl and $C H_{3} COOH$ , $\land_{m}$ increases (and κ decreases) on dilution. [1 mark]
Difference: for KCl (strong electrolyte) $\land_{m}$ increases only slightly and linearly with $c$ , so $\land_{m}^{o}$ can be found by extrapolation; for $C H_{3} COOH$ (weak electrolyte) $\land_{m}$ is much lower and rises steeply at high dilution, so $\land_{m}^{o}$ cannot be obtained by extrapolation. [1 mark]

Answer: (a) fewer ions per unit volume; (b) α = 0.94; (c) more, H⁺ more mobile than K⁺; OR (c) similarity: Λm increases on dilution; difference: steep rise for CH₃COOH, gradual for KCl

Where the marks are

Fewer ions per unit volume on dilution (1 mark)
α = 141/150 = 0.94 (1 mark)
Λm more for HCl (1 mark)
Higher mobility of H⁺ than K⁺ (1 mark)
OR: one similarity (1 mark) and one difference (1 mark)

Revise the chapter

Chapter: Electrochemistry (notes, formulas, where marks are lost and every past question)

Practise this chapterBoard-style questions from Electrochemistry, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 56/5/1: every question, by section.

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CBSE Class 12 Chemistry 2023: question 32

Hints

Step-by-step solution

Revise the chapter

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