Chemistry A Molecular Approach AP Edition Chapter 12 Solutions 

12.1 Thirsty Solutions: Why You Shouldn't Drink Seawater 

• Seawater draws water out of the body 
• Seawater is a "thirsty" solution 
• Seawater draws water toward itself 
• A solution is a homogeneous mixture of two or more substances/components 
• The majority component is the solvent 
• The minority component is the solute 
• Solutions form because of intermolecular forces 
• Molecules want to form uniform mixtures over pure substances
• If a person drinks seawater, the water in their body will flow out in order to make the seawater uniform
• As the concentrations become more uniform, the solutions starts to dilute 

12.2 Types of Solutions and Solubility 

• Gaseous solutions are two gases mixed together 
• Liquid solutions can be a gas and a liquid, a liquid and a liquid, or a solid and a liquid 
• Solid solutions are two solids
• In aqueous solutions, water is the solvent and some other solid, liquid, or gas in the solute 
• Soluble means the solute will dissolve in the solvent 
• Insoluble means the solute will not dissolve in the solvent 
• Solubility is the amount of substance that will dissolve in a given solvent 
• Solubility depends on mixing and intermolecular forces 

Nature's Tendency toward Mixing: Entropy 

• Although most situations tend to lower potential energy, this is not true for all solutions 
• Mixing two ideal gases does not lower their potential energy 
• Entropy measures energy randomization or energy dispersal 
• Energy dispersal happens in gases 
• Energy dispersal happens when things are going from hot to cold in order to heat up the other end 

The Effect of Intermolecular Forces 

• Intermolecular forces can allow for or prevent solution formation 
• Intermolecular forces exist in three ways: Solvent-solute, Solvent-solvent, and Solute-solute
• Solvent-solute is the interaction between solvent and solute particles 
• Solvent-solvent is the interaction between a solvent particle and another solvent particle 
• Solute-solute is the interaction between a solute particle and another solute particle 
• Miscibility is the ability to be soluble in all proportions 
• If the solvent-solute interactions are greater than the solvent-solvent and solute-solute interactions a solution will form 
• If the solvent-solute interactions are equal to the solvent-solvent and solute-solute interactions a solution will form 
• If the solvent-solute interactions are less than the solvent-solvent and solute-solute interactions a solution may or may not form 
• Solution formation does depend on disparity 
• The general rule of thumb is that "like dissolves like" 
• Polar solvents tend to dissolve polar or ionic solutes 
• Nonpolar solvents tend to dissolve nonpolar solutes 

12.3 Energetics of Solution Formation 

• When heat is released the process is exothermic (-) 
• When heat is absorbed the process is endothermic (+) 
• Separating a solute is always endothermic because energy is required to break IM forces 
• Separating a solvent is always endothermic because energy is required to break IM forces
• Mixing solute particles with solvent particles is always exothermic because energy is released when interactions occur 
• The enthalpy of a solution is the sum of the enthalpy changes in each step 
• If the sum of the endothermic processes is equal to the exothermic process the enthalpy is 0 
• If the endothermic processes are less than the exothermic process the enthalpy will be negative. The solution will be exothermic 
• If the endothermic processes are greater than the exothermic process the enthalpy will be positive. The solution will be endothermic 
• If the enthalpy of a solution is too large a solution will not form 

Aqueous Solutions and Heats of Hydration 

• The heat of hydration is the enthalpy change that occurs when 1 mole of the gas solute dissolves in water 
• The enthalpy of hydration is always largely negative for ionic compounds 
• If the enthalpy of the solute is less than the enthalpy of hydration, the enthalpy of the solution is negative. The solution will be exothermic
• If the solution is exothermic the solution will feel warm to the touch 
• If the enthalpy of the solute is greater than the enthalpy of hydration, the enthalpy of the solution is positive. The solution will be endothermic 
• If the solution is endothermic the solution will feel cool to the touch 
• If the enthalpy of the solute is about equal to the enthalpy of hydration, the enthalpy of the solution is about 0. It will not be endo or exothermic 
• If the enthalpy of the solution is about 0, there is no noticeable change in temperature 

12.4 Solution Equilibrium and Factors Affecting Solubility 

• The dissolution of a solute in a solvent is an equilibrium process
• Initially, the rate of dissolution is faster than the rate of recrystallization 
• As the concentration of the dissolved solute increases, the rate of recrystallization increases 
• When the rate of dissolution and recrystallization are equal, dynamic equilibrium is reached 
• In a saturated solution all solute has been dissolved. If more solute were to be added, it would not dissolve 
• Saturated solutions are in dynamic equilibrium 
• In an unsaturated solution not all of the solute has been dissolved. If more solute was added it would dissolve 
• Unsaturated solutions are not at dynamic equilibrium 
• A supersaturated solution can contain more solute than it is supposed to 
• Supersaturated solutions are unstable  

The Temperature Dependence of the Solubility of Solids

• Solubility of a solid in water strongly depends on temperature 
• The solubility of most solids increases with an increase in temperature 
• Recrystallization is a way to purify a solid 
• Recrystallization is used to make rock candy 

Factors Affecting the Solubility of Gases in Water

• Solutions of gases dissolved in water are common
• The solubility of a gas in a liquid are temperature and pressure dependent

The Effect of Temperature 

• The solubility of gases in liquids decrease with an increase in temperature
• There is an inverse relationship between gas solubility and temperature 
• Warm temperature results in lower oxygen concentration 

The Effect of Pressure 

• The higher the pressure of a gas above a liquid, the more soluble the gas is in the liquid
• Henry's law quantifies the solubility of gas with increasing pressure 
• Sgas = kHPgas 
• Sgas is the solubility of the gas in M 
• kH is the constant of proportionality (Henry's law constant) 
• kH depends in the specific solute and solvent (also temperature) 
• Pgas is the partial pressure of the gas in atm 

12.5 Expressing Solution Concentration 

• A dilute solution contains a small amount of solute compared to the solvent 
• A concentrated solution contains large amounts of solute compared to the solvent 
• Concentration can be found with molarity, molality, parts by mass, parts by volume, mole fraction, and mole percent 

Molarity 

• The unit for molarity is M 
• M = amount of solute / volume of solution 
• The amount of solute is always in moles 
• The volume of the solution is always in L 
• Molarity is often used when making, diluting, or transerfing solutions 
• Molarity depends on volume. Since volume depends on temperature, molarity is temperature dependent 

Molality 

• Molality is showed with m 
• m = amount of solute / mass of solvent 
• The amount of solute is always in moles 
• The mass of the solvent is always in kg 
• Molality is used when comparing concentrations over a range of temperatures

Parts by Mass and Parts by Volume 

• Parts by mass is the ratio of the mass of solute to the mass of the solution. It is then multiplied by a multiplication factor
• (Mass solute / Mass solution) x multiplication factor 
• For percent by mass, the multiplication factor is 100 
• Percent by mass = (mass solute / mass solution) x 100% 
• Percent means per hundred. A solution with a 14% concentration contains 14g solute for 100g solution 
• Dilute solutions typically use parts per million (ppm) or parts per billion (ppb) 
• Parts per million have a multiplication factor of 10^6 
• Parts per billion has a multiplication factor of 10^9 
• ppm = (mass solute / mass solution) x 10^6 
• ppb = (mass solute / mass solution) x 10^9 
• Parts by volume is a ratio of volume of solute to volume of solution. It is then multiplied by a multiplication factor 
• (Volume solute / Volume solution) x multiplication factor 
• A solution with 22% volume has 22mL solute for 100mL solution 

Using Parts by Mass (or Parts by Volume) in Calculations 

• Parts by mass or parts by volume can be used as a conversion factor 

Mole Fraction and Mole Percent 

• Mole fraction is represented by Xsolute 
• Xsolute = amount solute / total amount of solute and solvent 
• nsolute / nsolute + nsolvent 
• Both are represented in moles 
• Mole percent is the mole fraction times 100% 
• mol % = Xsolute x 100% 

12.6 Colligative Properties: Vapor Pressure Lowering, Freezing Point Depression, Boiling Point Elevation, and Osmotic Pressure 

• Colligative properties depend on the number of particles dissolved in a solution 
• Colligative properties do not depend on the type of particle 
• Colligative properties include vapor pressure lowering, freezing point depression, boiling point elevation, and osmotic pressure 
• When 1 mole of a nonelectrolyte is dissolved in water, 1 mole of dissolved particles is formed 
• When 1 mole of an electrolyte is dissolved in water, more than 1 mole of dissolved particles is formed 

Vapor Pressure Lowering 

• Vapor pressure of a liquid is the pressure of gas above the liquid when they are in dynamic equilibrium 
• The vapor pressure of a the solution is lower than the vapor pressure of a pure solvent 
• When a nonvolatile solute is added, the rate of vaporization is slower than the rate of the pure solvent (rate if condensation is greater) 
• By the time the rates are equal again, the concentration of solvent molecules has decreased 
• A concentrated solution has the ability to draw a solvent to itself 
• Nature's tendency to mix causes a solution to become less and less concentrated 
• Raoult's law quantifies the vapor pressure of a solution 
• Psolution = XsolventPsolvent 
• Psolution is the vapor pressure of the solution 
• Xsolvent is the mole fraction of the solvent 
• Psolvent is the vapor pressure of the pure solvent at the same temperature 
• Vapor pressure lowering is the difference in vapore pressure between the pure solvent and the solution 
• Vapor pressure lowering = Psolvent - Psolution 
• The vapor pressure lowering is directly proportional to the mole fraction of the solute 

Vapor Pressure of Solutions Containing a Volatile (Nonelectrolyte) Solute 

• If a solution contains a volatile solvent and volatile solute, both affect the vapor pressure of the solution 
• An ideal solution will follow Raoult's law at all concentrations (for both solute and solvent)
• In an ideal solution the solute-solvent interactions are equal to solvent-solvent interactions 
• In a nonideal solution, solute-solvent interactions are either stronger or weaker than solvent-solvent interactions 
• If solute-solvent interactions are stronger, the solute will stop the solvent from vaporizing quickly 
• If a solution is dilute, Raoult's law works as an approximation 
• If a solution is not dilute, the effect and vapor pressure will be less than Raoult's predicted amount 
• If solute-solvent interactions are weaker, the solute will allow more vaporization 
• A solution that is not dilute will have a large effect and the vapor pressure will be greater than Raoult's predicted amount 

Freezing Point Depression and Boiling Point Elevation 

• Vapor pressure lowering occurs at all temperatures 
• A lower melting point is called freezing point depression 
• A higher boiling point is called boiling point elevation 
• The freezing point of a solution is lower than the freezing point of a pure solvent
• Tf = m X Kf
• Tf is the change in temperature of the freezing point in Celsius (usually a positive number) 
• m is molality of solution in moles solute per kilogram solvent 
• Kf is the freezing point depression constant for the solvent
• The boiling point of a solution is higher than the boiling point of a pure solvent 
• Tb = m X Kb 
• Tb is the change in temperature of the boiling point in Celsius 
• m is the molality of the solution in moles solute per kilogram solvent 
• Kb is the boiling point of elevation constant for the solvent 

Osmotic Pressure 

• Osmosis is the flow of solvent from a solution of a lower solute concentration to one of a higher solute concentration 
• Concentrated solutions draw solvent from more dilute solutions 
• The semipermeable membrane allows some substances to pass through, but not all 
• Osmotic pressure is the pressure required to stop osmotic glow 
• Osmotic pressure = MRT 
• M is molarity of the solution 
• T is the temperature (in Kelvin) 
• R is the ideal gas constant 

12.7 Colligative Properties of Strong Electrolyte Solutions 

• Van't Hoff factor is the ratio of moles or particles in a solution to moles of formula units dissolved 
• Van't Hoff factor is represented with and i 
• i = moles of particles in solution / moles of formula units dissolved 
• The expected factor only occurs in very dilute solutions 
• The expected values do not occur because some ions pair in a solution 
• To calculated the different properties for an ionic solution, multiple the m by i 
• Tf = im X Kf 
• Tb = im X Kb 
• Osmotic pressure = iMRT 

Strong Electrolytes and Vapor Pressure 

• An electrolyte solutions has a greater freezing point depression than a solution that is a nonelectrolyte solution of the same concentration 
• Vapor pressure of an electrolyte solution is greater than that of a nonelectrolyte solution

Colligative Properties and Medical Solutions 

• Osmotic pressures that are greater than body fluids are hyperosmotic 
• Hyperosmotic solutions take water out of cells and tissues 
• Osmotic pressures that are less than body fluids are hyposmotic 
• Hyposmotic solutions pump water into cells 
• Isosmotic solutions are normal

12.8 Colloids 

• Colloidal dispersion, or a colloid, is a mixture when the substance is dispersed evenly but not fully dissolved 
• Milk, fog, smoke, and whipped cream are all colloids 
• Colloids are determined by the size of the particles 
• If the diameter is between 1nm and 1000nm, the mixture is a colloid 
• Colloid particles move in what is called the Brownian motion 
• In micelles, the nonpolar end interacts with the center of a sphere to maximize interactions 
• Micelles structure scatter light 
• Scattering light is called the Tyndall effect