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Topic 7 Solutions. Solutions. A solution is a homogeneous mixture of substances in the same physical state Homogeneous means the particles are evenly distributed and spread uniformly throughout the mixture. Types of Solutions. Solutions can exist in all three states (s, l, g)

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Topic 7 Solutions


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    1. Topic 7 Solutions

    2. Solutions • A solution is a homogeneous mixture of substances in the same physical state • Homogeneousmeans the particles are evenly distributed and spread uniformly throughout the mixture

    3. Types of Solutions Solutions can exist in all three states (s, l, g) • gas - both solvent/solute are gases ex. Air • liquid - gas, liquid, or solid dissolved in a liquid ex. salt water, antifreeze • Miscible – ex. Sugar and water • Immiscible – ex. Oil and water • Solids - 2 or more solids are uniformly spread out • alloy- ex. brass = Cu/Zn; steel = iron + carbon • aqueous –water is the solvent, (aq) ex. NaCl(aq)

    4. SOLUTIONS 1. Solute - substance being dissolved • Are homogeneous mixtures are made up of two parts: Saltin salt water Carbon dioxide or sugar in soda drinks 2. Solvent - present in greater amount Waterin salt water Water is a common solvent Solutions made with water are called aqueous solutions (aq) Waterin soda

    5. Charateristics of liquid solutions • Homogeneous mixtures • Are clear and don’t disperse light • Can have color • Will not settle upon standing • Will pass through a filter

    6. Hydration • the process of dissolving a solid in water solute particles are surrounded by solvent particles First... solute particles are separated and pulled into solution Then...

    7. Dissolution of sodium Chloride Hydration

    8. Hydration process Dissociation: separation of an ionic solid into aqueous ions Molecular solvation: molecules stay intact; covalent C6H12O6(s)  C6H12O6(aq) NaCl(s)  Na+(aq) + Cl–(aq) Electrolyte –conducts Why? Ions present Non Electrolyte –doesn’t conduct Why? Ions are NOT present

    9. - + - - + + acetic acid salt sugar Solvation and Conductivity Non- Electrolyte Weak Electrolyte Strong Electrolyte solute exists as ions and molecules solute exists as ions only solute exists as molecules only DISSOCIATION

    10. Solubility • How much of a solute that will dissolve in a certain amount of solvent at a given temperature • Substances with a high solubility are soluble (aq) • Substances with a low solubility are insoluble (s) – they are precipitates • Use Reference Table F to determine

    11. “Like Dissolves Like” • used to describe what solutes • will dissolve in what solvent • depends on the forces of attraction Solute type nonpolar solvent (soap) polar solvent(water) Nonpolar(grease) soluble insoluble Polar (sugar) insoluble soluble Ionic (salt) insoluble soluble

    12. Solubility: Temperature • As temp increases, most solids become more soluble in water • As temp increases, most gases become less soluble in water Sol (s & l) temp Sol (g) temp

    13. Solubility: Pressure • Pressure has little or no effect on the solubility of solid or liquid solutes. Pressure does effect the solubility of gases in liquids • As press increases, the solubility of gases in a liquid increases • See Reference Table G Sol (g) pressure

    14. Factors Affecting Solubility • Temperature affects the solubility of solid, liquid, and gaseous solutes in a solvent; both temperature and pressure affect the solubility of gaseous solutes.

    15. Rate of Solution Formation • The composition of the solvent and the solute determine whether a substance will dissolve. • The factors that determine how fast (rate) a substance dissolves are • temperature • stirring (agitation) • the surface area of the dissolving particles

    16. Rate of Solution Formation • Temperature and Solution Formation • At higher temperatures, the kinetic energy of water molecules is greater than at lower temperatures, so they move faster. As a result, the solvent molecules collide with the surface of the sugar crystals more frequently and with more force.

    17. Temperatureand Solution Formation • Granulated sugar dissolves in cold water more quickly than a sugar cube, especially with stirring. • Granulated sugar dissolves very quickly in hot tea. A cube of sugar in cold tea dissolves slowly. • At higher temperatures, the kinetic energy of water molecules is greater than at lower temperatures, so they move faster. • As a result, the solvent molecules collide with the surface of the sugar crystals more frequently and with more force.

    18. Stirring and Solution Formation • Stirring speeds up the dissolving process because fresh solvent (the water in tea) is continually brought into contact with the surface of the solute (sugar). Particle Sizeand Solution Formation • A spoonful of granulated sugar dissolves more quickly than a sugar cube because the smaller particles in granulated sugar expose a much greater surface area to the colliding water molecules.

    19. Therefore… Solids tend to dissolve best when: • Heated • Stirred • Ground into small particles Gases tend to dissolve best when: • The solution is cold • Pressure is high

    20. Rate of Solution • Definitions: • Potential Energy (PE) – energy stored within the chemical bonds of a substance • Kinetic Energy (KE) – molecules in motion • Temperature – measurement of the average KE • direct relationship between the two temp = KE

    21. Saturation and Solubility Curves 1. Saturated solution – solution that cannot dissolve any more solute at a given temperature • added solute will NOT dissolve • At equilibrium: • Ratedissolving = Ratecrystallization • NaCl(aq)NaCl(s) 2. Unsaturated solution – solution that can dissolve more solute at a given temperature • added solute will dissolve – NOT at equilibrium 3. Supersaturated solution – solution that holds more solute than it can dissolve at a given temperature • produced by dissolving solute at a high temperature and allowing it to cool slowly • addition of solute causes precipitation of the excess

    22. Concentration – the amount of solute compared to solvent • Qualitative descriptions • concentrated – large amount of solute compared to the amount of solvent • example: frozen concentrated OJ • dilute – small amount of solute compared to the amount of solvent • example: weakcoffee Concentrated vs. Dilute

    23. UNSATURATED SOLUTION more solute dissolves SATURATED SOLUTION no more solute dissolves SUPERSATURATED SOLUTION becomes unstable, crystals form Solubility – the ability to dissolve in water concentration

    24. Solubility Curves • shows the dependence of solubility on temperature

    25. Solubility Curves • shows the dependence of solubility on temperature • maximum grams of solute that will dissolve in 100 g of solvent at a given temperature • varies with temp • based on a saturated solution

    26. Saturation –vs- Unsaturated A solution that contains the maximum amount of solute in a given amount of solvent. A solution that contains less solute than a saturated solution under existing conditions is unsaturated.

    27. Supersaturated • A solution that contains more dissolved solute than a saturated solution under the same conditions is supersaturated. • The crystallization of a supersaturated solution can be initiated if a very small crystal, called a seed crystal, of the solute is added.

    28. Supersaturated • A supersaturated solution is clear before a seed crystal is added.

    29. Supersaturated • Crystals begin to form in the solution immediately after the addition of a seed crystal.

    30. Excess solute crystallizes rapidly.

    31. Practice Question #1 1) According to Reference Table G, which compound solubility decreases most rapidly as the temperature changes from 10°C to 70°C? a) NH4Cl b) NH3 c) HCl d) KCl

    32. Answer to problem #1 • Correct Answer Number: 2 • Explanation: See Ref. Table G. Notice the curves for choices 1 and 4 increase. Choice 2 and 3 both decrease but choice 2 (NH3) decreases more than 50 degrees, while HCl decreases only about 16 degrees.

    33. Concentrations of Solutions • Water must be tested continually to ensure that the concentrations of contaminants do not exceed established limits. These contaminants include metals, pesticides, bacteria, and even the by-products of water treatment. You will learn how solution concentrations are calculated.

    34. Quantities in Solutions • The amount of solute in a solution. • Describing Concentration • % by mass - medicated creams • % by volume - rubbing alcohol • ppm, ppb - water contaminants • molarity - used by chemists • molality - used by chemists

    35. Molarity • Molarity (M) is the number of moles of solute dissolved in one liter of solution. • To calculate the molarity of a solution, divide the moles of solute by the volume of the solution.

    36. Molarity • To make a 0.25 molar (0.25M) solution, first add ? mol of solute to a 0.50-L (500mL) volumetric flask half filled with distilled water.

    37. Solve for Moles • Solute used is NaCl • We want to prepare 500 mL of a 0.25 Molar solution • Using the Molarity formula solve for moles of solute M = Moles of Solute Liters of Soln

    38. Plug in Numbers • 0.250 mol/Liter = x moles 0.500L # of Moles = 0.250mol/liter x 0.500 L x = 0.125 moles

    39. Convert Moles to Grams • 0.125 moles NaCl ? Grams • Use Mole Conversion Formula • Moles = given mass gfm

    40. Plug in Numbers • 0.125 moles = x grams 58 g/mol (gfm of NaCl) X= 7.25 grams NaCl

    41. Molarity • Add 7.25 grams of NaCl to water in flask • Swirl the flask carefully to dissolve the solute.

    42. 16.2 Molarity • Fill the flask with water exactly to the 500-mL mark.

    43. #1: How many moles of NaOH are contained in 200 ml of 0.1M solution of NaOH? • Answer: • moles = (M)(L) • = (0.1 M)(.2L) • =.02 mol

    44. Answer: M = (moles)/(L) = 1.50 moles .5000L = 3.00 M Ex #2: What is the molarity of a 500.0 mL solution of NaOH (GFM = 40.0g) with 60.0 g of NaOH (aq)?

    45. Different style problem 1) Which solution is the most concentrated? a) 1 mole of solute dissolved in 1 liter of solution? b) 2 moles of solute dissolved in 3 liters of solution? c) 6 moles of solute dissolved in 4 liters of solution? d) 4 moles of solute dissolved in 8 liters of solution?

    46. Answer to different style problem 1) Which solution is the most concentrated? a) 1 mole of solute dissolved in 1 liter of solution? b) 2 moles of solute dissolved in 3 liters of solution? c) 6 moles of solute dissolved in 4 liters of solution? d) 4 moles of solute dissolved in 8 liters of solution? • Correct Answer Number: 3 • Explanation: #3 (1.5 M. ) is the most concentrated. Find the molarity ( moles of solute/liter of solution) for each answer. #1) 1 mole / 1 liter or 1 M.        #2) 2 moles / 3 liters or 0.67 M. #3) 6 moles / 4 liters or 1.5 M.       #4) 4 moles / 8 liters or 0.5 M.

    47. 16.2 Percent Solutions • Concentration in Percent (Mass/Mass)

    48. Concentration = Mass of solute (g) Volume of Solvent or Solution (mL) • Substitute “volume” for “mass” in the above equation and vice versa. • Ex.#1 What is the percent by volume of hexane if 20.0 mL of hexane are dissolved in benzene to a total volume of 80.0 mL? % volume = volume of solute x 100 volume of solution = 20.0 mL x 100 80.0 mL = 25.0%

    49. A 50.0 gram sample of a solution is evaporated and found to contain 0.100 grams of sodium chloride. What is the percent by mass of sodium chloride in the solution? % mass = 0.100 g X 100 50.0 g = 0.200%

    50. 100.0 grams of water is evaporated and analyzed for lead. 0.00010 grams of lead ions are found. What is the concentration of the lead, in parts per million? • ppm = grams of solute x 1,000,000 • grams of solution • = (0.00010 g) x 1,000,000 • 100.00010 g • = .9 ppm • If the legal limit for lead in the water is 3.0 ppm, then the water sample is within the legal limits (it’s safe and ok)