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 \cm^{2} \mol^{-1} 1. 1.00 111.3 \\times10^{-3} 111.3 2. 0.10 12.9 \\times10^{-3} 129.0 3. 0.01 1.41 \\times10^{-3} 141.0 Answer the following questions: (a) Why does conductivity decrease with dilution? (b) If {\∧^\circ_{m}} of {KCl} is 150.0 {\S} {\cm} {\mol}^{-1} , calculate the degree of dissociation of 0.01 {\M} {\KCl} . (c) If Rahul had used{HCl}instead to {KCl} then would you except the \∧_{{m}} values to be more or less than those per {KCl} for a given concentration. Justify. OR (c) Amit, a classmate of Rahul repeated the same experiment with {CH}_3 {COOH} solutioninsteadof {KCl} solution. Give one point that would be similar and one that would be different in his observations as compared to Rahul.

CBSE Class 12 Chemistry 2023 Delhi Set 1 Solved Paper

Question : 32 of 35

Marks: +1, -0

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 $^{-1}$	$∧_m\ \S\ \cm^{2}\ \mol^{-1}$
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

Answer the following questions:
(a) Why does conductivity decrease with dilution?
(b) If

{\∧°_{m}}

{KCl}

150.0 {\S} {\cm} {\mol}^{-1}

, calculate the degree of dissociation of

0.01 {\M} {\KCl}

.
(c) If Rahul had used

{HCl}

instead to

{KCl}

then would you except the

\∧_{{m}}

values to be more or less than those per

{KCl}

for a given concentration. Justify.
OR
(c) Amit, a classmate of Rahul repeated the same experiment with

{CH}_3 {COOH}

solutioninsteadof

{KCl}

solution. Give one point that would be similar and one that would be different in his observations as compared to Rahul.

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)

\∝={i°_m }/{i_m}

i_m=150.0 {\S} {\cm}^2{\mol}^{-1}

i°_m =141.0 {\S} {\cm}^2 {\mol}^{-1}

\∝={141}/{150}=0.94

{HCl}

will be high because when it break down into ions, it produce

{HCl} \→{H}^{+}+{Cl}^{-}

Its

{H}^{+}

(cation) size is smallerthen

{K}^{+}

ion. So for same concentration of

{HCl}

and

{KCl}, {HCl}

shows high molar conductivity.
[OR]
(c) (i)

{KCl}

is strong electrolyte and completely dissociate into their respective ion while

{CH}_3-{COOH}

is weak electrolyteand do not completely dissociate.
(ii) Number of ions produced after dissociation are equal

{KCl} \→{K}^{+}+{Cl}^{-}

{CH}_3-{COOH} \→{CH}_3-{COO}^{-}+{H}^{+}

Go to Question:

Prev Question

Next Question