CBSE 2023 · Region 5 · Set 1 · Q32 · 4 marks

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

S.No.	Conc.(M)	$\displaystyle k$ S cm$\displaystyle ^{-1}$	$\displaystyle \mathbf{\Lambda}_m$ S cm$\displaystyle ^2$ mol$\displaystyle ^{-1}$
1.	$\displaystyle 1.00$	$\displaystyle 111.3 \times 10^{-3}$	$\displaystyle 111.3$
2.	$\displaystyle 0.10$	$\displaystyle 12.9 \times 10^{-3}$	$\displaystyle 129.0$
3.	$\displaystyle 0.01$	$\displaystyle 1.41 \times 10^{-3}$	$\displaystyle 141.0$

Answer the following questions :

(a)

Why does conductivity decrease with dilution ?

(b)

If $\displaystyle \mathbf{\Lambda}_m^{\circ}$ of $\displaystyle \mathrm{KCl}$ is $\displaystyle 150.0$ S cm$\displaystyle ^2$ mol$\displaystyle ^{-1}$, calculate the degree of dissociation of $\displaystyle 0.01$ M $\displaystyle \mathrm{KCl}$.

(c)OR(c)

If Rahul had used $\displaystyle \mathrm{HCl}$ instead to $\displaystyle \mathrm{KCl}$ then would you expect the $\displaystyle \mathbf{\Lambda}_m$ values to be more or less than those per $\displaystyle \mathrm{KCl}$ for a given concentration. Justify.

CBSE Class 12 Chemistry past-paper question from the 2023board exam, with the answer as CBSE’s own marking scheme gives it. Where our answers come from.