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Chapter 18 Solutions

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

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  1. Chapter 18Solutions Milbank High School

  2. Section 18.1Properties of Solutions • OBJECTIVES: • Identify the factors that determine the rate at which a solute dissolves.

  3. Section 18.1Properties of Solutions • OBJECTIVES: • Calculate the solubility of a gas in a liquid under various pressure conditions.

  4. 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?

  5. 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.

  6. 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)

  7. 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

  8. 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

  9. 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)

  10. 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 =

  11. 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

  12. Section 18.2Concentration of Solutions • OBJECTIVES: • Solve problems involving the molarity of a solution.

  13. Section 18.2Concentration of Solutions • OBJECTIVES: • Describe how to prepare dilute solutions from more concentrated solutions of known molarity.

  14. Section 18.2Concentration of Solutions • OBJECTIVES: • Explain what is meant by percent by volume [ % (v/v) ], and percent by mass [ % (m/v) ] solutions.

  15. 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

  16. 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

  17. 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?

  18. 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?

  19. Dilution Adding water to a solution

  20. 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

  21. 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?

  22. 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?

  23. 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?

  24. 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.

  25. 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.

  26. Colligative Properties Depend only on the number of dissolved particles Not on what kind of particle

  27. 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

  28. 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.

  29. 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.

  30. Section 18.4Calculations Involving Colligative Properties • OBJECTIVES: • Calculate the molality and mole fraction of a solution.

  31. 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.

  32. 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?

  33. 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.

  34. 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.

  35. 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?

  36. 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 =

  37. 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

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

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