Chapter 15 - SolutionsChapter 15 - Solutions

A solution is a homogeneous A solution is a homogeneous mixture of two or more substances mixture of two or more substances in a single physical state.in a single physical state.

Examples: air, brass, salt water, Examples: air, brass, salt water, carbon dioxide in watercarbon dioxide in water

Miscible - pairs of liquids that will Miscible - pairs of liquids that will mixmix

Immiscible - pairs of liquids that Immiscible - pairs of liquids that will not mixwill not mix

Aqueous solutions - solutions in Aqueous solutions - solutions in which water is the solventwhich water is the solvent

Concentration - the amount of Concentration - the amount of solute in a given amount of solventsolute in a given amount of solvent

Molarity (M) = mol of solute/L of Molarity (M) = mol of solute/L of solutionsolution

Demonstrate sample problem 1 Demonstrate sample problem 1 on page 507on page 507

Assignment: C-15 page 507 Assignment: C-15 page 507 practice problems 1 & 2practice problems 1 & 2

Molality (m) = mol of solute/Kg of Molality (m) = mol of solute/Kg of solventsolvent

1 L of water = 1 kg of water1 L of water = 1 kg of water

1 mL of water = 1 g1 mL of water = 1 g

Demonstrate sample problem Demonstrate sample problem page 508page 508

Assignment: C-15 page 508 Assignment: C-15 page 508 practice problems 3 & 4practice problems 3 & 4

Describe how you would Describe how you would make each of the following.make each of the following.

150 mL of .27 M NaOH solution150 mL of .27 M NaOH solution

Start by finding how many moles of Start by finding how many moles of NaOH this will require and then NaOH this will require and then change that to grams. Mix this with change that to grams. Mix this with water to dissolve the NaOH and then water to dissolve the NaOH and then continue to add water until the total continue to add water until the total volume of the solution is 150 mL. volume of the solution is 150 mL.

Approx. 150 mL of .27 m NaOH Approx. 150 mL of .27 m NaOH solutionsolution

This will require the same number of This will require the same number of grams of NaOH as above, only now grams of NaOH as above, only now we will just add this to 150 mL of we will just add this to 150 mL of water. The final volume will not be water. The final volume will not be exactly 150 mL, however the solution exactly 150 mL, however the solution will be .27 m.will be .27 m.

Mole fraction = mol of solute/total mol of Mole fraction = mol of solute/total mol of solutionsolutionIf 2 moles of water is mixed with 1 mole of If 2 moles of water is mixed with 1 mole of sugar and 1 mole of alcohol.sugar and 1 mole of alcohol.The mole fraction of sugar is .25 and the The mole fraction of sugar is .25 and the mole fraction of water is .5. mole fraction of water is .5. Demonstrate sample problem 3 on page Demonstrate sample problem 3 on page 510.510.Assignment: C-15 page 510 practice Assignment: C-15 page 510 practice problems 5 & 6problems 5 & 6

Unsaturated - less solute than the Unsaturated - less solute than the solution can hold under existing solution can hold under existing conditions.conditions.

Saturated - as much solute as the Saturated - as much solute as the solution can possibly hold solution can possibly hold at the at the current temperature and pressurecurrent temperature and pressure

Supersaturated - more solute that Supersaturated - more solute that the solution can hold (some will the solution can hold (some will precipitate out)precipitate out)

The Dissolving ProcessThe Dissolving ProcessWhen something dissolves, bonds When something dissolves, bonds between solute molecules are between solute molecules are broken (which require energy) and broken (which require energy) and new bonds with water are formed new bonds with water are formed (which give off energy). (which give off energy).

If breaking the solute bonds takes If breaking the solute bonds takes more energy than bonding with more energy than bonding with water molecules gives back then the water molecules gives back then the overall dissolving process will overall dissolving process will require energy (endothermic) and require energy (endothermic) and the heat of solution (∆H) will be the heat of solution (∆H) will be positive.positive.

If the solute - water bonds provide If the solute - water bonds provide more energy than it takes to more energy than it takes to break the bonds between solute break the bonds between solute molecules then the dissolving molecules then the dissolving process will give off heat process will give off heat (exothermic) and the heat of (exothermic) and the heat of solution (∆H) will be negative.solution (∆H) will be negative.

Solubility - The three factors Solubility - The three factors that affect solubility are:that affect solubility are:

the nature of the solvent and solute the nature of the solvent and solute (like dissolves like) (like dissolves like)

temperature - The solubility of most temperature - The solubility of most gases decrease with temperature. gases decrease with temperature. The solubility of most solids The solubility of most solids increases with temperature.increases with temperature.

pressure - The solubility of gases pressure - The solubility of gases increases with increasing pressure.increases with increasing pressure.

See solubility graphs on pages 516 See solubility graphs on pages 516 and 517.and 517.

The factors that affect the The factors that affect the rate of dissolving are:rate of dissolving are:

The surface area of the solute. The surface area of the solute. Making a powder increases the Making a powder increases the surface area.surface area.

Stirring. Stirring brings less Stirring. Stirring brings less concentrated solvent in contact concentrated solvent in contact with the solute.with the solute.

Temperature. Increasing Temperature. Increasing temperature speeds up molecule temperature speeds up molecule movement.movement.

Colligative Properties - properties Colligative Properties - properties that are dependent on the number that are dependent on the number of solute particles but independent of solute particles but independent of their chemical identity. of their chemical identity.

Four colligative propertiesFour colligative properties

vapor pressure reduction - As the vapor pressure reduction - As the mole fraction of the solute increases, mole fraction of the solute increases, the vapor pressure of the the vapor pressure of the solvent decreases. solvent decreases.

boiling point elevation - This is boiling point elevation - This is related to vapor pressure reduction. related to vapor pressure reduction. A liquid boils when its vapor pressure A liquid boils when its vapor pressure is equal to the atmospheric pressure. is equal to the atmospheric pressure. Since the overall vapor pressure is Since the overall vapor pressure is lowered, it takes a higher lowered, it takes a higher temperature to increase the vapor temperature to increase the vapor pressure enough to equal the pressure enough to equal the atmospheric pressure.atmospheric pressure.

∆∆TTbb = K = Kbbmm (see page 522 for K(see page 522 for Kbb))

Demonstrate sample problem 4 Demonstrate sample problem 4 on page 523on page 523

Assignment: C-15 page 523 Assignment: C-15 page 523 practice problems 7 & 8practice problems 7 & 8

freezing point depression - ∆Tfreezing point depression - ∆Tff = K= Kffm m

(see page 526)(see page 526)

d)d) Osmotic Pressure - Osmotic Osmotic Pressure - Osmotic pressure is due to the fact that water pressure is due to the fact that water tends to move through a tends to move through a semipermeable membrane from semipermeable membrane from the side of lower solute concentration the side of lower solute concentration to the side of higher concentration. to the side of higher concentration.