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Colligative Properties Part 1

Colligative Properties Part 1. Colligative Properties of Solutions. Colligative properties of a solution depend only on the concentration of solute particles, and not on the nature of the solute. Non-colligative properties: color, odor, viscosity, toxicity, reactivity, etc.

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Colligative Properties Part 1

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  1. Colligative Properties Part 1

  2. Colligative Properties of Solutions • Colligative propertiesof a solution depend only on the concentration of solute particles, and not on the nature of the solute. • Non-colligative properties: color, odor, viscosity, toxicity, reactivity, etc. • We will examine four colligative properties: • Vapor pressure (of the solvent in a solution) • Freezing point depression • Boiling point elevation • Osmotic pressure

  3. Vapor Pressure of a Solution • The vapor pressure of solvent above a solution is less than the vapor pressure above the pure solvent. • Raoult’s law: the vapor pressure of the solvent above a solution (Psoln) is the product of the vapor pressure of the pure solvent (Posolv) and the mole fraction of the solvent in the solution (xsolv): Psoln = xsolv . Posolv

  4. Practice 1 • The vapor pressure of water at 20⁰ C is 2.33 kPa.  A 20% w/w solution of ethylene glycol in water is prepared.  Assuming that the solute is non-volatile, do a calculation to estimate the vapor pressure of the solution.

  5. Practice 2 • Pentane and heptane are two hydrocarbon liquids present in gasoline.  At 20oC, the vapor pressure of pentane is 56.00 kPa and the vapor pressure of heptane is 4.80 kPa.  What is the total vapor pressure of a solution prepared with a concentration of 30.0 mole percent pentane in heptane?

  6. A Vapor-Pressure Related Phenomenon …the volume of B will decrease and its concentration will increase, AND… Begin with solution A (higher concentration of solute) and solution B (lower concentration). Solution ___ has the higher vapor pressure. …the volume of A increases and its concentration drops, UNTIL… Solvent evaporates more readily from B than from A. Condensation occurs at the same rate for both. Therefore… …the concentrations of A and B are the same.

  7. Vapor Pressure LoweringBy A Nonvolatile Solute At all temperatures, the vapor pressure (curve) from a solution (nonvolatile solute) is lower than… This means that the solid-liquid curve must also shift to the left. …the vapor pressure from the pure solvent. Consequences: the boiling point of the solution increases by DTb … …and the freezing point decreases by DTf .

  8. Freezing Point Depression and Boiling Point Elevation DTf= Kfm DTb= Kbm

  9. Practice 3 • Glycerol, C3H8O3, (formula weight 92) , is essentially a nonvolatile liquid that is very soluble in water.  A solution is made by dissolving 46.0 g of glycerol in 250 g of water.  Calculate the boiling point of the solution at an atmospheric pressure of 1 atm.

  10. Practice 4 • A solution of 14 g of an unknown (molecular) compound in 1 kg of benzene boiled at 81.7oC.  Calculate the formula weight of the unknown.

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