JEE Advanced 2016 · Paper 2 · Chemistry Q20

← JEE Advanced 2016 · Paper 2

ChemistrySingle correct+3 / -1Question 20 of 54

The qualitative sketches I, II and III given below show the variation of surface tension with molar concentration of three different aqueous solutions of KCl, $\ce{CH3OH}$ and $\ce{CH3(CH2)11OSO3^-Na+}$ at room temperature. The correct assignment of the sketches is

Three plots of surface tension against concentration. I: falls gradually from the start and levels off. II: rises very slightly, nearly flat. III: drops steeply at low concentration and then stays flat at a low value.

I: KCl; II:

\ce{CH3OH}

; III:

\ce{CH3(CH2)11OSO3^-Na+}

\ce{CH3(CH2)11OSO3^-Na+}

; II:

\ce{CH3OH}

; III: KCl

I: KCl; II:

\ce{CH3(CH2)11OSO3^-Na+}

; III:

\ce{CH3OH}

\ce{CH3OH}

; II: KCl; III:

\ce{CH3(CH2)11OSO3^-Na+}

Answer and worked solution

Official answer: D

KCl: an inorganic electrolyte that is depleted at the surface. It raises surface tension very slightly. That is the nearly flat, slightly rising sketch II.
CH₃OH: mildly surface-active. It lowers surface tension gradually. That is I.
Sodium dodecyl sulphate: a surfactant. Surface tension drops sharply until the critical micelle concentration, then stays flat. That is III.

(D)

Topics:Colloids Soaps and detergents

← Previous question Next question →

JEE Advanced 2016 · Paper 2, Chemistry question 20