JEE Main 2022 · 25 Jun · Shift 2 · Chemistry Q63

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ChemistrySingle correct+4 / -1Question 63 of 90

Solute A associates in water. When 0.7 g of solute A is dissolved in 42.0 g of water, it depresses the freezing point by 0.2 °C. The percentage association of solute A in water, is :

[Given: Molar mass of A = 93 g mol $^{-1}$ . Molal depression constant of water is 1.86 K kg mol $^{-1}$ .]

50%

60%

70%

80%

Answer and worked solution

Official answer: D

Molality. $m=\dfrac{0.7/93}{0.042}\approx0.179$ mol kg $^{-1}$ .

van 't Hoff factor. $i=\dfrac{\Delta T_f}{K_fm}=\dfrac{0.2}{1.86\times0.179}\approx0.6$ .

Dimerisation. $i=1-\dfrac\alpha2$ , so $\alpha=0.8$ : 80% association.

(D)

Topics:van't Hoff factor Colligative properties

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JEE Main 2022 · 25 Jun · Shift 2, Chemistry question 63