JEE Advanced 2019 · Paper 1 · Chemistry Q3

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ChemistrySingle correct+3 / -1Question 21 of 54

Molar conductivity $(\Lambda_m)$ of aqueous solution of sodium stearate, which behaves as a strong electrolyte, is recorded at varying concentrations $(c)$ of sodium stearate. Which one of the following plots provides the correct representation of micelle formation in the solution?

(critical micelle concentration (CMC) is marked with an arrow in the figures)

Λm against √c: a straight line falling until the CMC, then horizontal beyond it.

Λm against √c: a line rising to a peak at the CMC, then falling.

Λm against √c: a straight line falling gently until the CMC, then falling more steeply.

Λm against √c: a line falling to a minimum at the CMC, then rising.

Answer and worked solution

Official answer: C

Below the CMC. Sodium stearate is a strong electrolyte, so by Kohlrausch's law $\Lambda_m=\Lambda_m^\circ-A\sqrt c$ : a straight line falling with $\sqrt c$ .

Above the CMC. The stearate ions aggregate into micelles. This sharply reduces the number of free ions and their mobility, so $\Lambda_m$ falls more steeply.

That is the falling line with a steeper second branch, plot (C).

Topics:Colloids Conductance and Kohlrausch's law

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JEE Advanced 2019 · Paper 1, Chemistry question 3