2. Colligative Properties
Depend only on the number, not on the identity, of the solute particles in an ideal solution:
Boiling-point elevation
Freezing-point depression
Osmotic pressure
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3. Boiling-Point Elevation
Nonvolatile solute elevates the boiling point of the solvent.
ΔT = Kbmsolute
ΔT = boiling-point elevation
Kb = molal boiling-point elevation constant
msolute = molality of solute
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4. Boiling Point Elevation: Liquid/Vapor Equilibrium
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5. Boiling Point Elevation: Addition of a Solute
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6. Boiling Point Elevation: Solution/Vapor Equilibrium
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7. Freezing-Point Depression
When a solute is dissolved in a solvent, the freezing point of the solution is lower than that of the pure solvent.
ΔT = Kfmsolute
ΔT = freezing-point depression
Kf = molal freezing-point depression constant
msolute = molality of solute
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8. Freezing Point Depression: Solid/Liquid Equilibrium
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9. Freezing Point Depression: Addition of a Solute
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10. Freezing Point Depression: Solid/Solution Equilibrium
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11. Changes in Boiling Point and Freezing Point of Water
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12. EXERCISE!
A solution was prepared by dissolving g of glucose in g water. The molar mass of glucose is g/mol. What is the boiling point of the resulting solution (in °C)? Glucose is a molecular solid that is present as individual molecules in solution.
The change in temperature is ΔT = Kbmsolute. Kb is 0.51 °C·kg/mol. To solve formsolute, use the equation m = moles of solute/kg of solvent.
Moles of solute = (25.00 g glucose)(1 mol / g glucose) = mol glucose
Kg of solvent = (200.0 g)(1 kg / 1000 g) = kg water
msolute = ( mol glucose) / ( kg water) = mol/kg
ΔT = (0.51 °C·kg/mol)( mol/kg) = 0.35 °C.
The boiling point of the resulting solution is °C °C = °C.
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13. EXERCISE!
A plant cell has a natural concentration of m. You immerse it in an aqueous solution with a freezing point of –0.246°C. Will the cell explode, shrivel, or do nothing?
The cell will explode (or at least expand). The concentration of the solution is m. Thus, the cell has a higher concentration, and water will enter the cell.
Note: Use the red box animation to assist in explaining how to solve the problem.
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14. van’t Hoff Factor, i
The relationship between the moles of solute dissolved and the moles of particles in solution is usually expressed as:
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15. Ion Pairing
At a given instant a small percentage of the sodium and chloride ions are paired and thus count as a single particle.
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16. Examples
The expected value for i can be determined for a salt by noting the number of ions per formula unit (assuming complete dissociation and that ion pairing does not occur).
NaCl i = 2
KNO3 i = 2
Na3PO4 i = 4
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17. Ion Pairing Ion pairing is most important in concentrated solutions.
As the solution becomes more dilute, the ions are farther apart and less ion pairing occurs.
Ion pairing occurs to some extent in all electrolyte solutions.
Ion pairing is most important for highly charged ions.
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18. Modified Equations
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