JEE Main 2017 · 2 Apr · Shift 1 · Chemistry Q37

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ChemistrySingle correct+4 / -1Question 37 of 90

The freezing point of benzene decreases by 0.45°C when 0.2 g of acetic acid is added to 20 g of benzene. If acetic acid associates to form a dimer in benzene, percentage association of acetic acid in benzene will be :

( $K_f$ for benzene = 5.12 K kg mol $^{-1}$ )

74.6%

94.6%

64.6%

80.4%

Answer and worked solution

Official answer: B

Molality $=\dfrac{0.2/60}{0.020}=0.1667$ . Then $i=\dfrac{0.45}{5.12\times0.1667}=0.527$ .

For dimerisation $i=1-\frac\alpha2$ , so $\alpha=0.946$ : 94.6% association.

Topics:van't Hoff factor Colligative properties

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JEE Main 2017 · 2 Apr · Shift 1, Chemistry question 37