JEE Main 2026 · 2 Apr · Shift 1 · Chemistry Q55

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ChemistrySingle correct+4 / -1Question 55 of 75

19.5 g of fluoro acetic acid (molar mass = 78 g mol $^{-1}$ ) is dissolved in 500 g of water at 298 K. The depression in the freezing point of water was 1 °C. What is $K_a$ of fluoro acetic acid ? (For water, $K_f=1.86$ K kg mol $^{-1}$ ). Assume molarity and molality to have same values.

10^{-6}

4\times10^{-4}

3\times10^{-5}

3\times10^{-3}

Answer and worked solution

Official answer: D

Molality. $\frac{19.5/78}{0.5}=0.5$ mol kg⁻¹.

Van 't Hoff factor. $i=\dfrac{\Delta T_f}{K_fm}=\dfrac1{1.86\times0.5}=1.075$ , so the degree of dissociation is $\alpha=0.075$ .

Ka. $K_a=\dfrac{c\alpha^2}{1-\alpha}=\dfrac{0.5(0.075)^2}{0.925}\approx3\times10^{-3}$ .

(D)

Topics:van't Hoff factor pH and ionisation Colligative properties

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JEE Main 2026 · 2 Apr · Shift 1, Chemistry question 55