Read the passage carefully and answer the questions that follow:
Rahul set up an experiment to find the resistance of aqueous KCl solution for different concentrations at 298 K using a conductivity cell connected to a Wheatstone bridge, fed with A.C. power in the audio frequency range. From the resistance at the null point he calculated the conductivity (κ) and molar conductivity (Λ_m):
S.No | Conc. (M) | κ (S cm⁻¹) | Λ_m (S cm² mol⁻¹)
1 | 1.00 | 111.3 × 10⁻³ | 111.3
2 | 0.10 | 12.9 × 10⁻³ | 129.3
3 | 0.01 | 1.41 × 10⁻³ | 141.0
(a) Why does conductivity decrease with dilution?
(b) If Λ°_m 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 of KCl, would you expect the Λ_m values to be more or less than those for KCl at a given concentration? Justify.
OR
(c) Amit, a classmate of Rahul, repeated the same experiment with CHCOOH 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's.
Rahul set up an experiment to find the resistance of aqueous KCl solution for different concentrations at 298 K using a conductivity cell connected to a Wheatstone bridge, fed with A.C. power in the audio frequency range. From the resistance at the null point he calculated the conductivity (κ) and molar conductivity (Λ_m):
S.No | Conc. (M) | κ (S cm⁻¹) | Λ_m (S cm² mol⁻¹)
1 | 1.00 | 111.3 × 10⁻³ | 111.3
2 | 0.10 | 12.9 × 10⁻³ | 129.3
3 | 0.01 | 1.41 × 10⁻³ | 141.0
(a) Why does conductivity decrease with dilution?
(b) If Λ°_m 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 of KCl, would you expect the Λ_m values to be more or less than those for KCl at a given concentration? Justify.
OR
(c) Amit, a classmate of Rahul, repeated the same experiment with CHCOOH 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's.
Solution
(a) Conductivity is the conductance of ions present in a unit volume. On dilution the number of ions per unit volume decreases, so κ decreases.
(b)
(c) More. The H ion has an exceptionally high ionic mobility (conductance) compared with K, so HCl's Λ at any concentration is much higher than KCl's.
OR
(c) Similar: for CHCOOH too, Λ increases on dilution.
Different: being a weak electrolyte, its κ and Λ values are much smaller, and Λ rises steeply at very low concentration (increasing dissociation) — so Λ cannot be obtained by extrapolation, unlike for KCl.
(b)
(c) More. The H ion has an exceptionally high ionic mobility (conductance) compared with K, so HCl's Λ at any concentration is much higher than KCl's.
OR
(c) Similar: for CHCOOH too, Λ increases on dilution.
Different: being a weak electrolyte, its κ and Λ values are much smaller, and Λ rises steeply at very low concentration (increasing dissociation) — so Λ cannot be obtained by extrapolation, unlike for KCl.
CBSE Class 12 Board · 2023 · Subjective