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Chapter 18 Solutions . Milbank High School. Section 18.1 Properties of Solutions. OBJECTIVES: Identify the factors that determine the rate at which a solute dissolves. Section 18.1 Properties of Solutions. OBJECTIVES:

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Chapter 18 solutions l.jpg

Chapter 18Solutions

Milbank High School


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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 can’t

    • 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...

  • Henry’s 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

  • Aconcentrated solution has a large amount of solute

  • Adilute 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 doesn’t 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= Kbx 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= -Kfx 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.


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