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Solutions — Chemistry STD 12 Science — Question

Rajasthan BoardEnglish MediumSTD 12 ScienceChemistrySolutions3 Marks
Question
  1. Urea forms an ideal solution in water. Determine the vapour pressure of anaqueous solution containing 10% by mass of urea at 40°C. (Vapour pressure of water at 40°C = 55.3 mm of Hg)
  2. Why is freezing point depression of 0.1 M sodium chloride solution nearly twice that of 0.1 M glucose solution?
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Answer

  1. $\frac{\text{P}^{\circ}_{A}-\text{P}_{A}}{\text{P}^{\circ}_{A}}=\frac{\text{Moles of Solvent}}{\text{Moles solute + Moles of solvent}}$
$\text{P}^{\circ}_{\text{A}}=55.3\text{ mm of Hg, P}_{\text{A}}=?$
Mass of Solvent = (100 - 10)g = 90g,
No. of Moles of solvent =$\frac{\text{90g}}{\text{18g mol}^{-1}}=\text{5 mol} $
No. of Moles of solute$\frac{\text{10}}{\text{60}}=\frac{\text{1}}{\text{6}}\text{ mol}$
$\frac{\text{55.3 - P}_{A}}{\text{55.3}}=\frac{\frac{1}{6}}{\frac{1}{6}+5}=\frac{\text{1}}{\text{31}}$
$\text{P}_{A}\text{=53.52 mm Hg}.$
  1. Sodium chloride dissolves in water to form two ions whereas glucose is non–electrolyte and remains in molecular form only.

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