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JEE Mains · Chemistry · STD 12 -1. Solution and colligative properties

\(19.5\) 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.

  1. A \(10^{-6}\)
  2. B \(4 \times 10^{-4}\)
  3. C \(3 \times 10^{-5}\)
  4. D \(3 \times 10^{-3}\)
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Answer & Solution

Correct Answer

(D) \(3 \times 10^{-3}\)

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Detailed explanation

The molality of the solution is given by: \(m = \dfrac{\text{Mass of solute}}{\text{Molar mass of solute} \times \text{Mass of solvent in kg}}\) \(m = \dfrac{19.5}{78 \times 0.5} = 0.5 \text{ mol kg}^{-1}\) The depression in freezing point is given by:…
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