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Solutions VII
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Relative lowering in vapour pressure and elevation in boiling point

Shaillee Kaushal is teaching live on Unacademy Plus

Shaillee Kaushal
Faculty in Chemistry with a teaching experience of 14 years. Specialised in teaching for boards and competitions.

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  1. XII - 02 Solution VII


  2. COLLIGATIVE PROPERTIES: o Lowering in vapour pressure o Elevation in boiling point o Depression in freezing point o Osmotic pressure


  3. Relative lowering in vapour pressure: If Po is the vapour pressure ofpure solvent and Ps is the vapour pressure of the solutionthen According to Raoult's law: Since B' is a non volatile solute no vapour pressure is contributed by B 0 Substracting both sides by 1, we have 0 The term Po- Lowering in vapour pressure This is not a colligative property The term 0- Relaive lowering in vapour pressure = This is a colligative property now nB nA+nB WBXMA


  4. If the vapour pressure ofa liquid is knownat a temperature, it can be calculated at other temperature using Clausius-Clapeyronequation log P Pressure at temperature T va 2.030R T T2 B = Pressure at temperature T2


  5. Elevation in boiling point: Ebullioscopy AT, boiling point of the solution - Boiling point of pure solvent From Raoult's law of dilute solutlion For pure solvent P, and MA are constant MBXWA MBXWA


  6. Molal elevation can be calculated by the following relation 2 RTb b 1000 Lv R - gas constant E boiling point of the solvent Lv = latent heat of vaporisation