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The vapor pressure of solutions

The vapor pressure of solutions. The presence of a nonvolatile solute inhibits the escape of solvent molecules from the liquid and so lowers the vapor pressure of the solvent. Raoult’s Law. The presence of a nonvolatile solute lowers the vapor pressure of a solvent.

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The vapor pressure of solutions

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  1. The vapor pressure of solutions

  2. The presence of a nonvolatile solute inhibits the escape of solvent molecules from the liquid and so lowers the vapor pressure of the solvent.

  3. Raoult’s Law The presence of a nonvolatile solute lowers the vapor pressure of a solvent. Psoln= XsolventPosolvent Psoln= vapor pressure of the solution Xsolvent= mole fraction of the solvent Posolvent= vapor pressure of the pure solvent

  4. For a solution that obeys Raoult's law, a plot of Psoln versus xsolvent gives a straight line.Slope=Po (VP of pure solvent) Can be used to determine the molar mass of solute or can give information about the number of ions Example: NaCl vs. Sugar

  5. Take now a solution of 2 volatile components:When a solution contains two volatile components, both contribute to the total vapor pressure.

  6. Ideal Solution of 2 volatile components obeys the modified Raoult’s Law: Psoln= XsolventPosolvent + XsolutePosolute IDEAL POSITIVE deviation NEGATIVE dev.

  7. Examples of Ideal vs. Nonideal Solutions • Example of Benzene-Toluene IDEAL (A…A  B…B) • Example of Ethanol-Hexane POSITIVE DEVIATION (A…A, B…B >> A…B; H>0) • Example of Water-Acetone NEGATIVE DEVIATION ((A…A, B…B << A…B; H<0)

  8. Nonideal Solutions Psoln> or < Pideal= XA.POA+ XBPOB

  9. Colligative Properties Depend only on the number, not on the identity, of the solute particles in an ideal solution. • Vapor pressure (nonvolatile solute) • Boiling point elevation • Freezing point depression • Osmotic pressure

  10. Phase diagrams for pure water (red lines) and for an aqueous solution containing a nonvolatile solute (blue lines).

  11. Boiling Point Elevation A nonvolatile solute elevates the boiling point of the solvent. T = Kbmsolute Kb = molal boiling point elevation constant m = molality of the solute

  12. Freezing Point Depression A nonvolatile solute depresses the freezing point of the solvent. T = Kfmsolute Kf = molal freezing point depression constant m = molality of the solute

  13. (a) Ice in equilibrium with liquid water. (b) Ice in equilibrium with liquid water containing a dissolved solute (shown in pink). Freezing point of solution is when first crystal appears • Remember: • at the melting point- Vapor pressure of solid= vapor pressure of liquid • Adding a solute to water lowers its vapor pressure • therefore: at 0oC it is all liquid at 1 atm, and water will not freeze at 0oC • but the VP of solid decreases with T more rapidly • at a temperature<0 for water, VP of liquid becomes = VP solid • resulting in a freezing point depression

  14. The addition of antifreeze lowers the freezing point of water in a car’s radiator.

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