JEE Main 2022 · 26 Jun · Shift 1 · Chemistry Q82

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A 0.5 percent solution of potassium chloride was found to freeze at $-0.24$ °C. The percentage dissociation of potassium chloride is ______. (Nearest integer)

(Molal depression constant for water is 1.80 K kg mol $^{-1}$ and molar mass of KCl is 74.6 g mol $^{-1}$ )

Your answer

Answer and worked solution

Official answer: 98

Molality. 0.5 g KCl in 99.5 g water: $m=\dfrac{0.5/74.6}{0.0995}\approx0.0674$ mol kg $^{-1}$ .

van 't Hoff factor. $i=\dfrac{0.24}{1.80\times0.0674}\approx1.98$ .

Dissociation. $i=1+\alpha$ , so $\alpha\approx0.98$ : 98%.

Answer: 98

Topics:van't Hoff factor Colligative properties

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JEE Main 2022 · 26 Jun · Shift 1, Chemistry question 82