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CBSE Class 12 Chemistry 2023 Delhi Set 1 Solved Paper

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Question : 32 of 35
Marks: +1, -0
Rahul set-up an experiment to find resistance of aqueous KCl\mathrm{KCl} solution for different concentrations at 298 K298\ \mathrm{K} using a conductivity cell connected to a Wheatstone bridge. He fed the Wheatstone bridge with a.c. power in the audio frequency range 550550to 5000 cycles per second. Once the resistance was calculated from null point he also calculated the conductivity KK and molar conductivity Λm\Lambda_m and recorded his readings in tabular form.
 S.No.  Conc.(M)  k S cm −1^{-1}   Λm S cm2 mol−1\Lambda_m\ \mathrm{S\ cm^{2}\ mol^{-1}}
 1.  1.00   111.3×10−3111.3 \times 10^{-3}  111.3
 2.  0.10   12.9×10−312.9 \times 10^{-3}  129.0
 3.  0.01   1.41×10−31.41 \times 10^{-3}  141.0
Answer the following questions:
(a) Why does conductivity decrease with dilution?
(b) If Λm∘{\Lambda^{\circ}_{m}} of KCl\mathrm{KCl} is 150.0 S cm mol−1150.0\ \mathrm{S\ cm\ mol^{-1}} , calculate the degree of dissociation of 0.01 M KCl0.01\ \mathrm{M\ KCl} .
(c) If Rahul had usedHCl\mathrm{HCl}instead to KCl\mathrm{KCl} then would you except the Λm{\Lambda_{m}} values to be more or less than those per KCl\mathrm{KCl} for a given concentration. Justify.
OR
(c) Amit, a classmate of Rahul repeated the same experiment with CH3COOH\mathrm{CH_3COOH} solutioninsteadof KCl\mathrm{KCl} solution. Give one point that would be similar and one that would be different in his observations as compared to Rahul.
Solution:     👈: Video Solution
(a) Conductivity decreases with dilution because it depends upon the number of ions present in the solution. When dilution increases number of available ions decreases. Hence, conductivity decreases.
(b) ∝=im∘im\propto = \frac{i^{\circ}_{m}}{i_m}
im=150.0 S cm2 mol−1i_m = 150.0\ \mathrm{S\ cm^2\ mol^{-1}}
im∘=141.0 S cm2 mol−1i^{\circ}_m = 141.0\ \mathrm{S\ cm^2\ mol^{-1}}
∝=141150=0.94\propto = \frac{141}{150} = 0.94
(c) Molar conductivity of HCl\mathrm{HCl} will be high because when it break down into ions, it produce
HCl→H++Cl−\mathrm{HCl} \rightarrow \mathrm{H}^{+} + \mathrm{Cl}^{-}
Its H+\mathrm{H}^{+}(cation) size is smallerthen K+\mathrm{K}^{+}ion. So for same concentration of HCl\mathrm{HCl} and KCl, HCl\mathrm{KCl},\ \mathrm{HCl} shows high molar conductivity.
[OR]
(c) (i) KCl\mathrm{KCl} is strong electrolyte and completely dissociate into their respective ion while CH3−COOH\mathrm{CH_3-COOH} is weak electrolyteand do not completely dissociate.
(ii) Number of ions produced after dissociation are equal
KCl→K++Cl−\mathrm{KCl} \rightarrow \mathrm{K}^{+} + \mathrm{Cl}^{-}
CH3−COOH→CH3−COO−+H+\mathrm{CH_3-COOH} \rightarrow \mathrm{CH_3-COO}^{-} + \mathrm{H}^{+}
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