Properties of Solutions Chapter 13

The Solution Process Section 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

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

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

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

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

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

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

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

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?

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

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

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

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

Calculating Mass Percent 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)

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

Molality Calculations 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)

Conversion of Concentration Units 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 (cont.) 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)

Colligative Properties Section 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

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:

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)?

Raoult's Law Calculations 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)

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:

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.

Calculation of Freezing-Point Depression 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 Boiling Point Elevation 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)

Freezing Point Depression in Aqueous Solutions 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)

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)

Determination of Molar Mass via Osmotic Pressure 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)