CBSE Class 12 · 2003 · All India · SolutionsShort Answer

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When does the measurement of colligative properties of a solution lead to abnormal molecular masses? The freezing point depression of 0.1 m NaCl solution is 0.372°C. What conclusion would you draw about the molecular state of NaCl in the solution? (K

_{f}

for water is 1.86 K kg mol

^{- 1}

)

Solution

Colligative properties count particles; when the solute dissociates or associates in solution, the particle number differs from the formula units dissolved, and the molar mass calculated from the colligative property comes out abnormal.
Expected depression for 0.1 m (no dissociation):

Δ T_{f}^{c a l c} = 1.86 \times 0.1 = 0.186 K

van't Hoff factor:

i = \frac{0.372}{0.186} = 2

i = 2

means each NaCl gives two particles —

NaCl is completely dissociated into Na^{+} and Cl^{-}

CBSE Class 12 Board · 2003 · All India · Subjective

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