Chapter 12 Solutions. Solution Composition. Types of Solutions: Physical State of Solute and Solvent. mol solute. volume solution

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1 Chapter 12 Solutions Aqueous Solutions What did we learn in Chapter 4? solute + solvent! solution speciation? stoichiometry? empirical solubility rules: Which ionic compounds are soluble in water? electrolyte definitions: strong electrolytes - soluble ionic compounds strong acids & strong bases weak electrolytes - weak acids & weak bases nonelectrolytes - molecular compounds insoluble ionic compounds Solution Composition representation of how much solute is present Types of Solutions: Physical State of Solute and Solvent qualitatively: concentrated vs. dilute quantitatively: molarity or molar concentration (units mol/l) mol solute M = volume solution

2 Solution Formation with a Molecular Solute Gas Phase Solution Formation speciation molecular compounds dissolve to produce solutions containing dispersed, individual, discrete, neutral molecules soluble molecular compounds are nonelectrolytes Ionic Compounds - Soluble or Insoluble? Solution Formation with an Ionic Solute speciation ionic compounds dissolve to produce solutions containing dispersed cations and anions must consider the compound s stoichiometry to fully understand the speciation lattice energy energy required to separate an ionic solid into its constituent gas phase ions NaCl (s)! Na+ (g) + Cl- (g) soluble ionic compounds are strong electrolytes lattice energy increases with increasing ionic charge and decreasing ionic radius in general - the greater the lattice energy, the lower the solubility

3 Saturated vs. Unsaturated Solutions Thermodynamics of Solution Formation sol n formation may be endothermic or exothermic overall: Hsoln = H1 + H2 + H3 when solutions form, there is an increase in disorder increase in entropy solution formation is a spontaneous process Solution Composition representation of how much solute is present qualitatively: concentrated vs. dilute quantitatively: molarity or molar concentration (units mol/l) mol solute M = volume solution Solution Composition mass percent composition: mass component mass of solution mass % component = x 100 related to mass percent composition: mass component mass of solution parts per million (ppm) = x 10 6 mass component mass of solution parts per billion (ppb) = x 10 9

4 Solution Composition mole fraction composition,! (unitless): mol component total mol of all components! of component = if solution is composed of 2 substances A and B:!A +!B = 1 Solution Composition molality or molal concentration (units mol/kg): mol solute kg solvent molality, m = calculations that involve relating a solution s molar concentration and molal concentration will require information about the density of the solution A solution is prepared by dissolving 13.5 g glucose (C6H12O6) in kg of water. Calculate the mass % glucose in this solution, and the molality of the solution. What is the molal concentration of a solution prepared by dissolving 36.5 g C10H8 in 425 g C7H8? What is the mole fraction of C10H8 (! C10H8 ) in this solution? A 2.5 g sample of groundwater contains 5.4 μg of Zn 2+. Calculate the ppm of Zn 2+ is the sample. A solution of HCl (aq) is 36% HCl by mass, and has a density of g/ml. Determine the molality and molarity of this solution.

5 Speciation and Solution Composition Factors Affecting Solubility nature and strength of solute & solvent interactions like dissolves like temperature pressure Solute-Solvent Interactions relative strengths of attractive forces between solute and solvent particles determines if a solution will form polar solute + polar solvent dipole-dipole force potential for H-bonding forces nonpolar solute + nonpolar solvent London dispersion forces polar solutes dissolve best in polar solvents nonpolar solutes dissolve best in nonpolar solvents like dissolves like

6 Miscible and Immiscible Liquids liquids that are miscible mix together in all proportions; infinite solubility liquids that are immiscible do not dissolve in one another; zero solubility Hydrophilic vs. Hydrophobic Compounds water-soluble compounds: hydrophilic (water loving) more polar functionality vitamin C water-insoluble compounds: hydrophobic (water fearing) more hydrocarbon (nonpolar) functionality hydrophobic compounds are lipophilic (fat souble) vitamin A Predict whether each of the following will be more soluble in CCl4 or C2H5OH: Effect of Temperature on Solubility the solubility of most solid solutes increases as temperature increases C7H16 Na2SO4 HCl I2 the solubility of gases decreases as temperature increases

7 Effect of Pressure on Solubility the solubility of a gas increases with increasing pressure relationship between P and solubility is given by Henry s Law: C = kp C = concentration of dissolved gas k = Henry s Law constant P = partial pressure of gas over solution Calculate the concentration of CO2 (g) in a carbonated drink bottled under PCO2 = 4.0 atm at 25 C. For CO2 (g) in water at 25 C, k = mol/l atm. Then - calculate the concentration of CO2 (g) in the drink after opening if PCO2 decreases to 3.0 x 10 4 atm. Properties of Solutions How do the following properties change when a solute is dissolved in a solvent? vapor pressure freezing point boiling point osmotic pressure We will be discussing colligative properties - dependent on the collection of particles present. We will need to consider the nature of solutes used: volatile or nonvolatile? electrolyte or nonelectrolyte? The addition of a nonvolatile solute to a solvent results in a solution with a lower vapor pressure (Psol n) than the vapor pressure of the pure solvent (P solv). Raoult s Law: Vapor Pressure of Solutions: Raoult s Law Psol n = (! solv )(P solv) as amount of solute increases: mol solute increases! solute increases! solvent decreases Psol n decreases

8 Ideal Solutions vs. Nonideal Solutions an ideal solution follows Raoult s Law nonideal solutions deviate from Raoult s Law behavior positive deviations weaker solutesolvent interactions see higher than predicted Psol n negative deviations stronger solutesolvent interactions see lower than predicted Psol n Calculate the vapor pressure of a solution prepared by dissolving 50.0 ml of glycerin in ml water at 25 C. some details: glycerin, C3H8O3 is a nonvolatile solute molar mass = g/mol d = 1.26 g/ml for water at 25 C, P vap = 23.8 torr molar mass = g/mol d = 1.00 g/ml Solution with 2 Volatile Components Boiling Point Elevation & Freezing Point Depression each component will follow Raoult s law: PA =! A P A and PB =! B P B Ptot = PA + PB =! A P A +! B P B Consider a mixture of 1.0 mol C6H mol C7H8. Determine the vapor pressure of this solution. For C6H6, P = 75 torr; for C7H8, P = 22 torr.

9 Boiling Point Elevation & Freezing Point Depression the boiling point of a solution is higher than the boiling point of the pure solvent Set-up for Freezing Point Depression Experiment: Tf = Kf m Tb = Kb m the freezing point of a solution is lower than the freezing point of the pure solvent Tf = Kf m A solution is prepared by dissolving 42.0 g eucalyptol (C10H18O, molar mass = g/mol) in kg chloroform (CHCl3). Determine the boiling point and freezing point of this solution. For CHCl3, Kb = 3.63 C/m; bp = 61.2 C; Kf = 4.68 C/m; fp = 63.5 C.

10 Osmotic Pressure Osmosis is the net movement of solvent molecules through a semi-permeable membrane from a region of lower concentration to higher concentration. results in volume in turn there is a change in P exerted on membrane osmotic pressure - pressure required to stop osmosis some terminology: Osmotic Pressure, V = nrt OR = (n/v)rt = MRT isotonic solutions solutions with the same osmotic pressure (and, same M at constant T) hypotonic solution solution with a lower and M hypertonic solution solution with a higher and M The average osmotic pressure of blood at 25 C is 7.70 atm. Determine the molar concentration of C6H12O6 (aq) that is isotonic with blood. What is the molar concentration of NaCl (aq) that is isotonic with blood at 25 C? Using Solution Properties for Determination of Molar Mass of a Solute g of a nonvolatile nonelectrolyte solute is dissolved in 40.0 g CCl4. The boiling point for the resulting solution is determined by experiment to be C. Determine the molar mass of the solute. for CCl4: bp = C; Kb = 5.02 C/m

11 Using Solution Properties for Determination of Molar Mass of a Solute 3.50 mg of a protein is dissolved in enough water to produce 5.00 ml of solution. The measured osmotic pressure of the solution is 1.54 torr at 25 C. Determine the molar mass of the protein. Properties of Electrolyte Solutions properties of solutions depend on the total concentration of solute particles: solution m C6H12O6 (aq) m NaNO3 (aq) m K2SO4 (aq) particle concentration expected fp observed fp m C C m C C m C C note: observed fp of electrolyte solutions is less than the expected (calculated) fp Properties of Electrolyte Solutions the observed difference in properties of electrolyte solutions is attributed to ion pairing total ion concentration is actually slightly lower than predicted based on stoichiometric considerations of complete dissociation of compound van t Hoff factor, i gives a measure of the extent of electrolyte dissociation mol particles i = mol solute solute Properties of Electrolyte Solutions theoretical value of i observed i 0.10 m observed i m observed i m C6H12O NaCl MgSO K2SO note: i closer to theoretical value at lower concentrations; extent of ion pairing is less in more dilute solutions i closer to ideal value when ion charges are smaller extent of ion pairing is less between ions of lower charge