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Properties of Solutions

This chapter explores the properties of solutions, including solubility, the factors affecting solubility, concentration, and colligative properties such as vapor pressure lowering and freezing point depression.

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Properties of Solutions

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  1. Properties of Solutions Chapter 13

  2. The Solution ProcessSection 13.1 • A solution is formed when the solute becomes completely dispersed throughout the solvent • Solubility depends on two major factors: • Intermolecular forces • Natural tendency for objects to spread out as much as possible

  3. Solvation of NaCl • The solvation process occurs when the intermolecular forces between solvent-solute are greater than that of solute-solute:

  4. NaCl vs. Hexane • Why doesn’t NaCl dissolve in nonpolar solvents such as hexane, C6H14?

  5. Energy Changes and Solution Formation Hsoln = H1 + H2 + H3

  6. Saturated Solutions and SolubilitySection 13.2 A saturated solution exists in a state of equilibrium between solvated ions or molecules and the crystallized form

  7. Solubility • The solubility of a substance refers to the minimum amount of solute required to create a saturated solution at a given temperature • Ex: The solubility of NaCl in water is 35.7 g/ 100 mL of water at 0 ºC

  8. Supersaturation • A supersaturated solution can be created under certain conditions • Contains more solute than required to make a saturated solution

  9. Factors Affecting SolubilitySection 13.3 • The most common factors affecting solubility of compounds are: • Solute-Solvent interactions • Pressure Effects (Only for gases) • Temperature Effects

  10. Solute-solvent Interactions The stronger the intermolecular forces between solute and solvent (as opposed to solute-solute), the greater the solubility • Ex: NaCl/H2O vs. C6H12O6/H2O • I2/H2O vs. I2/Benzene • What intermolecular forces exist and which have the greatest magnitude?

  11. Solutions of Liquids (Miscibility) • Essentially follows the rule that "like dissolves like" • Dipole-dipole interactions between solvent and solute determine the miscibility of liquids • butanol/H2O  soluble • diethyl ether/H2O  only slightly soluble

  12. Pressure and Temperature Effects • The solubility of gases increases with increasing pressure • Explains "the bends" for SCUBA divers • Expressed mathematically via Henry's Law: Solubility for all phases of matter increases with increasing temperature

  13. Expressing ConcentrationSection 13.4 • Concentration is most frequently expressed as moles/L (M) • Other quantitative expressions: • mass % • ppm • ppb • mole fraction • molality (mol solute/kg solvent)

  14. Mass Percentage & ppm • Mass percent is always expressed as follows: • ppm is very similar to mass percent, sometimes expressed as mg/L:

  15. A chemical analysis shows that 25.0 g of a solution contains 2.00 g glucose. Calculate the concentration in mass percentage of glucose. See Sample Exercise 13.4 (Pg. 542) Calculating Mass Percent

  16. Molality • Similar to molarity, but involves kg of solvent and not L • Typically only used when discussing colligative properties of solutions

  17. Determine the molality of a solution that contains 5.00 g of NaCl dissolved in 200 g of water. See Sample Exercise 13.5 (Pg. 544) Molality Calculations

  18. Ammonia is sold as a 24.5% aqueous solution. Express this concentration in both molality and mole fraction See Sample Exercise 13.6 (Pg. 544) Conversion of Concentration Units

  19. Determine the molarity of an aqueous HCl solution that is 2.00 molal. The density of this solution is 1.034 g/mL. See Sample Exercise 13.6 (Pg. 544) Conversion of Concentration Units (cont.)

  20. Colligative PropertiesSection 13.5 • Colligative properties of solutions include those properties that depend only on the quantity of solution and not what the solution is actually composed of • Ex: Lowering of vapor pressure, elevation of melting point, freezing point depression

  21. Raoult's Law • Introduction of a nonvolatile solute to a volatile solvent always results in a lowering of the vapor pressure for the new system • Expressed as:

  22. Solute Type • The type of solute that is used does not matter. It depends only on the total concentration of solute particles • Why does 1 mol NaCl lower the vapor pressure of water more than 1 mol of methanol (CH3OH)?

  23. Cyclohexane (C6H12) has a vapor pressure of 99.0 torr at 25 ºC. What is the vapor pressure (in torr) of cyclohexane above a solution of 14.0 g napthalene (C10H8) in 50 g cyclohexane at 25 ºC? See Sample Exercise 13.8 (Pg. 547) Raoult's Law Calculations

  24. Freezing Point Depression • For a pure liquid, intermolecular forces help align the molecules so that the phase change from liquid to solid can occur • Introduction of solute molecules disrupts this network and depresses the freezing point • This can be expressed mathematically as:

  25. Freezing-Point Depression/Boiling-Point Elevation • The equations shown below takes into account the number of dissolved particles per mole: • The variable i is referred to as the van’t Hoff factor • Represents the number of particles • Remember colligative properties depend only on the quantity of dissolved particles not the type.

  26. Pure ethylene freezes at 9.80 °C. A solution is made by dissolving 0.213 g of ferrocene (molar mass = 186.04 g/mol) in 10.0 g ethylene dibromide. The freezing-point depression constant, Kf for ethylene dibromide is 11.8 °C/molal. What is the freezing point of this solution? See Sample Exercise 13.9 (Pg. 550) Calculation of Freezing-Point Depression

  27. A solution is prepared by dissolving 1.00 g of a nonvolatile solute in 15.0 g acetic acid. The boiling point of this solution is 120.17 °C. The normal boiling point of acetic acid is 117.90 °C and it's boiling point elevation constant is 3.07 °C/molal. What is the molar mass of the solute? See Sample Exercise 13.9 (Pg. 550) Calculation of Boiling Point Elevation

  28. Arrange the following aqueous solutions in order of increasing freezing point: 0.05 m sucrose, 0.02 m NaCl, 0.01 m CaCl2, 0.03 m HCl See Sample Exercise 13.10 (Pg. 551) Freezing Point Depression in Aqueous Solutions

  29. Osmotic Pressure • Osmotic pressure is a colligative property that only applies to the processes involving semipermeable membranes • Solvent always flows to the area with the least amount of solute particles • Osmotic pressure is commonly used to determine the molecular weight of large molecules (i.e. proteins or polymers)

  30. Hemoglobin is a large molecule that carries oxygen in human blood. A water solution that contains 0.263 g of hemoglobin in 10.0 mL of solution has an osmotic pressure of 7.51 torr at 25 °C. What is the molar mass of hemoglobin? See Sample Exercise 13.13 (Pg. 556) Determination of Molar Mass via Osmotic Pressure

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