Chapter 18 Solutions

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Chapter 18Solutions

• Charles Page High School
• Dr. Stephen L. Cotton

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Section 18.1Properties of Solutions

• OBJECTIVES
• Identify the factors that determine the rate at
  which a solute dissolves.

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Section 18.1Properties of Solutions

• OBJECTIVES
• Calculate the solubility of a gas in a liquid
  under various pressure conditions.

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Solution formation

• Nature of the solute and the solvent
• Whether a substance will dissolve
• How much will dissolve
• Factors determining rate of solution...
• stirred or shaken (agitation)
• particles are made smaller
• temperature is increased
• Why?

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Making solutions

• In order to dissolve, the solvent molecules must
  come in contact with the solute.
• Stirring moves fresh solvent next to the solute.
• The solvent touches the surface of the solute.
• Smaller pieces increase the amount of surface
  area of the solute.

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Temperature and Solutions

• Higher temperature makes the molecules of the
  solvent move around faster and contact the solute
  harder and more often.
• Speeds up dissolving.
• Usually increases the amount that will dissolve
  (exception is gases)

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How Much?

• Solubility- The maximum amount of substance that
  will dissolve at a specific temperature (g
  solute/100 g solvent)
• Saturated solution- Contains the maximum amount
  of solute dissolved
• Unsaturated solution- Can still dissolve more
  solute
• Supersaturated- solution that is holding more
  than it theoretically can seed crystal will make
  it come out Fig. 18.7, p.506

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Liquids

• Miscible means that two liquids can dissolve in
  each other
• water and antifreeze, water and ethanol
• Partially miscible- slightly
• water and ether
• Immiscible means they cant
• oil and vinegar

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Solubility?

• For solids in liquids, as the temperature goes
  up-the solubility usually goes up (Fig. 18.4,
  p.504)
• For gases in a liquid, as the temperature goes
  up-the solubility goes down (Fig. 18.5, p.505)
• For gases in a liquid, as the pressure goes
  up-the solubility goes up (Fig. 18.6, p.505)

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Gases in liquids...

• Henrys Law - says the solubility of a gas in a
  liquid is directly proportional to the pressure
  of the gas above the liquid
• think of a bottle of soda pop, removing the lid
  releases pres.
• Equation S1 S2
• P1 P2

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Cloud seeding

• Ever heard of seeding the clouds to make them
  produce rain?
• Clouds- mass of air supersaturated with water
  vapor
• Silver Iodide (AgI) crystals are dusted into the
  cloud
• The AgI attracts the water, forming droplets to
  attract others

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Section 18.2Concentration of Solutions

• OBJECTIVES
• Solve problems involving the molarity of a
  solution.

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Section 18.2Concentration of Solutions

• OBJECTIVES
• Describe how to prepare dilute solutions from
  more concentrated solutions of known molarity.

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Section 18.2Concentration of Solutions

• OBJECTIVES
• Explain what is meant by percent by volume
  (v/v) , and percent by mass (m/v)
  solutions.

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Concentration is...

• a measure of the amount of solute dissolved in a
  given quantity of solvent
• A concentrated solution has a large amount of
  solute
• A dilute solution has a small amount of solute
• thus, only qualitative descriptions
• But, there are ways to express solution
  concentration quantitatively

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Molarity - most important

• The number of moles of solute in 1 Liter of the
  solution.
• M moles/Liter such as 6.0 molar
• What is the molarity of a solution with 2.0 moles
  of NaCl in 250 mL of solution?
• Sample 18-2, page 510

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Making solutions

• Pour in a small amount of solvent
• Then add the solute (to dissolve it)
• Carefully fill to final volume.
• Fig. 18-10, page 509
• Also M x L moles
• How many moles of NaCl are needed to make 6.0 L
  of a 0.75 M NaCl solution?

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Making solutions

• 10.3 g of NaCl are dissolved in a small amount of
  water, then diluted to 250 mL. What is the
  concentration?
• How many grams of sugar are needed to make 125 mL
  of a 0.50 M C6H12O6 solution?

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Dilution

• Adding water to a solution

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Dilution

• The number of moles of solute doesnt change if
  you add more solvent!
• The moles before the moles after
• M1 x V1 M2 x V2
• M1 and V1 are the starting concentration and
  volume.
• M2 and V2 are the final concentration and volume.
• Stock solutions are pre-made to known Molarity

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Practice

• 2.0 L of a 0.88 M solution are diluted to 3.8 L.
  What is the new molarity?
• You have 150 mL of 6.0 M HCl. What volume of 1.3
  M HCl can you make?
• Need 450 mL of 0.15 M NaOH. All you have
  available is a 2.0 M stock solution of NaOH. How
  do you make the required solution?

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Percent solutions...

• Percent means parts per 100, so
• Percent by volume Volume of solute x
  100 Volume of solution
• indicated (v/v)
• What is the percent solution if 25 mL of CH3OH is
  diluted to 150 mL with water?

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Percent solutions

• Percent by mass Mass of
  solute(g) x 100 Volume of
  solution(mL)
• Indicated (m/v)
• More commonly used
• 4.8 g of NaCl are dissolved in 82 mL of solution.
  What is the percent of the solution?
• How many grams of salt are there in 52 mL of a
  6.3 solution?

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Section 18.3Colligative Properties of Solutions

• OBJECTIVES
• Explain on a particle basis why a solution has a
  lower vapor pressure than the pure solvent of
  that solution.

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Section 18.3Colligative Properties of Solutions

• OBJECTIVES
• Explain on a particle basis why a solution has an
  elevated boiling point, and a depressed freezing
  point compared with the pure solvent.

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Colligative Properties

• Depend only on the number of dissolved particles
• Not on what kind of particle

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Vapor Pressure decreased

• The bonds between molecules keep molecules from
  escaping.
• In a solution, some of the solvent is busy
  keeping the solute dissolved.
• Lowers the vapor pressure
• Electrolytes form ions when they are dissolved
  more pieces.
• NaCl Na Cl- ( 2 pieces)
• More pieces bigger effect

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Boiling Point Elevation

• The vapor pressure determines the boiling point.
• Lower vapor pressure higher boiling point.
• Salt water boils above 100ºC
• The number of dissolved particles determines how
  much, as well as the solvent itself.

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Freezing Point Depression

• Solids form when molecules make an orderly
  pattern.
• The solute molecules break up the orderly
  pattern.
• Makes the freezing point lower.
• Salt water freezes below 0ºC
• How much depends on the number of solute
  particles dissolved.

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Section 18.4Calculations Involving Colligative
Properties

• OBJECTIVES
• Calculate the molality and mole fraction of a
  solution.

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Section 18.4Calculations Involving Colligative
Properties

• OBJECTIVES
• Calculate the molar mass of a molecular compound
  from the freezing point depression or boiling
  point elevation of a solution of the compound.

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Molality

• a new unit for concentration
• m Moles of solute kilogram of
  solvent
• m Moles of solute 1000 g of solvent
• What is the molality of a solution with 9.3 mole
  of NaCl in 450 g of water?

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Why molality?

• The size of the change in boiling point is
  determined by the molality.
• DTb Kb x m x n
• DTb is the change in the boiling point
• Kb is a constant determined by the solvent (Table
  18.2, page 523).
• m is the molality of the solution.
• n is the number of pieces it falls into when it
  dissolves.

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What about Freezing?

• The size of the change in freezing point is also
  determined by molality.
• DTf -Kf x m x n
• DTf is the change in freezing point
• Kf is a constant determined by the solvent (Table
  18.3, page 524).
• m is the molality of the solution.
• n is the number of pieces it falls into when it
  dissolves.

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Problems

• What is the boiling point of a solution made by
  dissolving 1.20 moles of NaCl in 750 g of water?
• What is the freezing point?
• What is the boiling point of a solution made by
  dissolving 1.20 moles of CaCl2 in 750 g of water?
• What is the freezing point?

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Mole fraction

• This is another way to express concentration
• It is the ratio of moles of solute to total
  number of moles of solute solvent (Fig. 18-19,
  p.522)
• na
• na nb

Sample 18-8, page 521
X
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Molar Mass

• We can use changes in boiling and freezing to
  calculate the molar mass of a substance
• Find 1) molality 2) moles, and then 3) molar
  mass
• Sample 18-10, page 524

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Key Equations

• Note the key equations on page 527 to solve
  problems in this chapter.