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Vapor Pressure Lowering

Vapor Pressure Lowering. Nonvolatile solutes reduce the ability of surface solvent molecules to escape the liquid. Therefore, vapor pressure is lowered. The amount of vapor pressure lowering depends upon the number of solute particles (a colligative property).

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Vapor Pressure Lowering

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  1. Vapor Pressure Lowering • Nonvolatile solutes reduce the ability of surface solvent molecules to escape the liquid. • Therefore, vapor pressure is lowered. • The amount of vapor pressure lowering depends upon the number of solute particles (a colligative property)

  2. In one regard, the affect on the vapor pressure of the solution depends the character of the SOLUTE Nonvolatile Francios Raoult 1830-1901 Raoult’s Law V.P.(sol’n) = C(solv) V.P.(solv)

  3. V.P.(sol’n) = C(solv) V.P.(solv) V.P. of pure solvent V. P. of the solution 0 1 Mole Fraction of Solvent

  4. Vapor Pressure Lowering…. SOLUTE Francios Raoult 1830-1901 Nonvolatile Volatile Non-ionizing Ionizing (modify mole fraction) Raoult’s Law (general form) V.P.(sol’n) = C(solv) V.P.(solv) V.P.(sol’n) = C(solv) V.P.(solv) + C(solute) V.P.(solute)

  5. A solution is prepared by mixing 50.0 grams of acetone (CH3COCH3) with 50.0 grams of methanol. The solution has a measured vapor pressure of 178.2 mm of Hg at 25.0°C. Does this solution obey Raoult’s Law? (i.e. is this an ideal solution) (note: the vapor pressures of pure acetone and methanol are 271 and 143 torr respectively at that temp.) V.P.(sol’n) = (0.356)(271 t)(acetone) + (0.645)(143 t)(meth) V.P. (sol’n) = 189 torr NOT IDEAL…..WHY? Negative Deviation!!!

  6. Do the particles like themselves more than they like each other or do they like each other more than they like themselves? A solution is prepared by mixing 50.0 grams of acetone (CH3COCH3) with 50.0 grams of methanol. The solution has a measured vapor pressure of 178.2 mm of Hg at 25.0°C. Does this solution obey Raoult’s Law? (i.e. is this an ideal solution) (note: the vapor pressures of pure acetone and methanol are 271 and 143 torr respectively at that temp.) They hold onto each other (better than to themselves) and inhibit the process of evaporation hence producing a lower V.P. than expected! V.P.(sol’n) = (0.356)(271 t)(acetone) + (0.645)(143 t)(meth) V.P. (sol’n) = 189 torr NOT IDEAL…..WHY? Negative Deviation!!!

  7. For a complete look then: SOLUTE Francios Raoult 1830-1901 Nonvolatile Volatile Non-Ionizing Ionizing (modify mole fraction) Ideal Non-Ideal DH=0 DH= - (neg. deviation) DH= + (pos. deviation) Raoult’s Law (general form) V.P.(sol’n) = C(solv) V.P.(solv) + C(solute) V.P.(solute)

  8. A solution is prepared by mixing 1.0 mol of benzene (C6H6) with 2.0 mol of toluene (CH3C6H5) at 20.0°C. • It is reasonable to assume ideal behavior here; why or why not? • What would the vapor pressure of the resulting solution be if the vapor pressures of pure benzene and toluene are 75 and 22 torr respectively at that temperature? • The vapor would be richer in which component? • Can you determine the composition of the vapor above the solution at equilibrium….??? Yes, DH≈ 0 (two non-polar molecules; none of the steps to solvation are large) V.P. = ⅓ (75 torr) + ⅔ (22 torr) = 40. torr The benzene, it contributes 25 torr of the overall 40. torr Dalton’s Law Pa = Pt Xa Yes, (25 t /40 t) = 0.63 = X(benzene)

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