JEE Advanced · Chemistry · 19. Surface Chemistry

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, $C H_{3} O H$ and $C H_{3} {(C H_{2})}_{11} O S O_{3}^{-} N a^{+}$ at room temperature. The correct assignment of the sketches is -

A $\text I : \text{KCl} , \text{II} : \text{CH} _3 \text{OH} , \text{III} : \text{CH} _3\left(\text{CH} _2\right)_{11}$$\text{OSO} _3^{-} \text{Na} ^{+}$
B $\text I : \text{CH} _3\left(\text{CH} _2\right)_{11} \text{OSO} _3^{-} \text{Na} ^{+}, \text{II} : \text{CH} _3 \text{OH},$ $\text{III} : \text{KCl}$
C $\text I : \text{KCl} , \text{II} : \text{CH} _3\left(\text{CH} _2\right)_{11} \text{OSO} _3^{-} \text{Na} ^{+}, \text{III} :$ $\text{CH} _3 \text{OH}$
D $\text I : \text{CH} _3 \text{OH} , \text{II} : \text{KCl} , \text{III} : \text{CH} _3\left(\text{CH} _2\right)_{11}$ $\text{OSO} _3^{-} \text{Na} ^{+}$

Verified Solution

Answer & Solution

Correct Answer

(D) $\text I : \text{CH} _3 \text{OH} , \text{II} : \text{KCl} , \text{III} : \text{CH} _3\left(\text{CH} _2\right)_{11}$ $\text{OSO} _3^{-} \text{Na} ^{+}$

Step-by-step Solution

Detailed explanation

Water has large surface tension due to very strong interaction. Generally adding organic derivatives to water decreases its surface tension due to hydrophobic interaction.
In case III, hydrophobic interaction is stronger than case II causing surface tension to decrease more rapidly.
Due to

K^{+} C l^{-}

(inorganic electrolyte) intermolecular forces increases, surface tension increases.

Apply the relevant identity from the chapter and substitute the values established in the previous step, keeping the original constraint in view.

Use the simplified expression to isolate the unknown, then plug the result back into the question setup to confirm the working is internally consistent.

Cross-check against a boundary or limiting case. Both the dimensional balance and the qualitative behaviour at the extreme should agree with the candidate answer.

Combine the steps above to identify the correct option. The remaining choices each fail at least one of the consistency, dimensional, or boundary checks.

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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, CH3OH and CH3CH211OSO3- Na+ at room temperature. The correct assignment of the sketches is -

Step-by-step Solution

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, $C H_{3} O H$ and $C H_{3} {(C H_{2})}_{11} O S O_{3}^{-} N a^{+}$ at room temperature. The correct assignment of the sketches is -