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A non-volatile solute is dissolved in a solvent of molar mass 50 g mol ⁻¹ . The vapour pressure of the pure solvent is 220 mm Hg and the vapour pressure of the solution is 200 mm Hg . The molality of the solution is:

Options

  1. A2 mol kg ⁻¹
  2. B1.82 mol kg ⁻¹
  3. C0.002 mol kg ⁻¹
  4. D200 mol kg ⁻¹

Correct answer

A. 2 mol kg ⁻¹

Step-by-step solution

For a solution containing a non-volatile solute, the relative lowering of vapour pressure can be related to the moles of solute and solvent as: P^ - P_ s P_ s = n_ solute n_ solvent We know that n_ solvent = W_ solvent M_ solvent , where W_ solvent is the mass of the solvent in grams and M_ solvent is its molar mass. The molality ( m ) of the solution is given by: m = n_ solute W_ solvent 1000 Substituting n_ solute W_ solvent = P^ - P_ s P_ s M_ solvent into the molality expression: m = ( P^ - P_ s P_ s ) 1000 M_

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