Chapter 11 Properties of Solutions Important Vocabulary • Homogeneous means there is only one phase (compositions do not vary) • Ex: Kool Aid, air, steel • Solute: Gets dissolved • Solvent: Does the dissolving • Solution: Homogeneous mixture consisting of a solute and solvent Dilute vs. Concentrated • Can’t be used in calculations • Molarity, mass percent, and mole fraction can be used to show solution concentrations Molarity • Moles of solute/liters of solution • Represented by M • Example: A solution was prepared by adding 5.84 g of formaldehyde, H2CO, to 100.0 g of water. The final volume of the solution was 104.0 mL. Calculate the molarity. • Answer: 1.87 M H2CO Mass Percent • Percent by mass of the solute in the solution • Mass Percent = (mass of solute/mass of solution) X 100% • Example: A solution was prepared by adding 5.84 g of formaldehyde, H2CO, to 100.0 g of water. The final volume of the solution was 104.0 mL. Calculate the mass percent. • Answer: 5.52 % H2CO, 94.48% H2O Mole Fraction • Represented by X • Moles of part/moles of solution X 100% Mole Frac. A = XA = nA/(nA+nB) • Example: A solution was prepared by adding 5.84 g of formaldehyde, H2CO, to 100.0 g of water. The final volume of the solution was 104.0 mL. Calculate the mole fraction. • Answer: XH2CO = 0.0338, XH2O = 0.9662 Molality • Represented by m • Moles of solute per kilogram of solvent Molality = moles of solute/kilogram of solvent • Example: A solution was prepared by adding 5.84 g of formaldehyde, H2CO, to 100.0 g of water. The final volume of the solution was 104.0 mL. Calculate the molality. • Answer: 1.94 m H2CO Solubility • Shows what will dissolve in what • “Like dissolves like” = polar solvents will dissolve polar/ionic solutes and nonpolar solvents will dissolve nonpolar solutes Factors Affecting Solubility 1. Structure 2. Pressure 3. Temperature 1. Structure Effects • Polarity of solute/solvent (like dissolves like) • Example: vitamins are fat-soluble and watersoluble – Fat-soluble = nonpolar, hydrophobic (water-fearing), build up/stored in fatty tissue, too much = hypervitaminosis – Water-soluble = polar, hydrophilic (water-loving), extra are excreted by the body 2. Pressure Effects • Doesn’t affect liquids/solids, but has a large affect on gases • Gas solubility increases as the partial pressure of the gas above the solution increases Henry’s Law • Shows relationship between gas pressure and concentration of dissolved gas: C = kP • C = concentration of dissolved gas • K = constant for particular solution • P = partial pressure of gas above solution • Works best with gases that don’t dissociate in/react with solvent Henry’s Law Example • The solubility of O2 is 2.2 X 10-4 M at 0C and 0.10 atm. Calculate the solubility of O2 at 0C and 0.35 atm. • Answer: 7.7 X 10-4 M O2 3. Temperature Effects • For most solids, solubility increases as temperature increases • For most gases, solubility decreases as temperature increases –Thermal Pollution in lakes: increase in temp. lowers dissolved oxygen concentrations Vapor Pressure of Solutions • If a solution contains a nonvolitile (not easily vaporized) solute, its vapor pressure is LOWER than the pure solvent. • Shells of water solvation make it so it’s harder for the solvent to vaporize • Molecules that do not dissociate (break up) in water (solvent) have higher vapor pressures than ionic compounds that do dissociate • The decrease in a solution’s vapor pressure is proportional to the number of particles the solute makes in solution. Answer This… • Which compound affects the vapor pressure of a solution the least: glucose, sodium chloride, or calcium chloride? • Solutions with covalent compounds > Solutions with ionic compounds Raoult’s Law • Calculates the expected vapor pressure of a solution based on the solute/solvent Psoln = XsolventP0solvent • Psoln = observed vapor pressure of solution • Xsolvent = mole fraction of solvent • P0solvent = vapor pressure of the pure solvent Example • Glycerin, C3H8O3, is a nonvolatile liquid. What is the vapor pressure of a solution made by adding 164 g of glycerin to 338 mL of H2O at 39.8C? The vapor pressure of pure water at 39.8C is 54.74 torr and its density is 0.992 g/mL. • Answer: 50.0 torr Example #2 • What is the vapor pressure of a solution made by adding 52.9 g of CuCl2, a strong electrolyte, to 800.0 mL of water at 52.0C? The vapor pressure of water is 102.1 torr, and its density is 0.987 g/mL. • Answer: 99.4 torr Colligative Properties • Depend on the number of solute particles, NOT their identity in an ideal solution 1. Boiling-Point Elevation 2. Freezing-Point Depression 3. Osmotic Pressure 1. Boiling-Point Elevation • Review boiling point definition: when vapor pressure = atmospheric pressure • When solute is added to solvent, it lowers the vapor pressure. • More kinetic energy must be added to bring the solution to boiling • Boiling point is HIGHER in solutions than in pure solvents • Antifreeze in car engines (solute) makes it so car engines don’t boil in high temperatures • The more solute particles dissolved, the higher the boiling point (identity doesn’t matter) Boiling-Point Elevation Calculation • Change in boiling point ∆Tb is the difference between the boiling point of the solution and the pure solvent • Unit: °C/m • Calculated using ∆Tb = Kb X msolute Kb is a molal boiling-point elevation constant of the solvent found on pg. 517 Example • What is the boiling point of a solution containing 96.7g of sucrose (C12H22O11) in 250.0g water at 1atm? • Answer: 100.579°C 2. Freezing-Point Depression • When solute is present, the normal molecular freezing pattern is disrupted • This makes it so the solution has to lose more kinetic energy (get colder) in order to solidify • Freezing point of the solution is LOWER than that of the pure solvent • The more solute particles dissolved, the more the freezing point decreases (identity doesn’t matter) • Sidewalk salt and car antifreeze work this way Freezing-Point Depression Calculation • Change in freezing point ∆Tf is the difference between the freezing point of the solution and the pure solvent • Unit: °C/m • Calculated using ∆Tf = Kf X msolute Kf is a molal freezing-point depression constant of the solvent found on pg. 517 Example • Determine the freezing point of a solution made by adding 27.5 g of methanol (CH3OH) to 250.0 g of water. • Answer: -6.39°C Example • Find the boiling point of a 1.50m solution of calcium chloride, CaCl2 and water. • Answer: 2.30°C so the new boiling point would be 102.30°C.