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Types of Solution: Concentration, Pressure, and Colligative Properties

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  1. Chapter 12 Solutions suroviec Spring 2015

  2. I. Types of Solution • Most chemical reaction take place between ions/molecules dissolved in water or a solvent. • Solution: homogenous mixture of 2 or more substances in a single phase

  3. II. Molecular view of solution process • Liquids and solids held together by intermolecular attractions • Looking at specifically • Solvent-solvent • Solvent - solute • Solute – solute • 3 steps • Separate solvent molecules • Separate solute molecules • Mix together

  4. Ex. Dissolve CaCl2 (s) in H2O • What happens to the concentration of Ca2+ and Cl-? • Saturated: stable solution in which the maximum amount of the solute has been dissolved • Equilibrium in solution • Solubility: concentration of solute in equilibrium with undissolved solute in a saturated solution

  5. Examples: • CCl4 and CH4 • NaCl and CCl4

  6. III. Concentration A. Molarity: (M) Moles of solute / liter of solution • Does not tell us the EXACT amount of volume to use B. Molality: (m) Mole of solute / mass of solvent (kg) • Cannot be the same measure of a solution

  7. Example The concentration of a K2CrO4 solution is undetermined, but it was prepared from 38.3 g of K2CrO4 (MW 19.4 g/mol) and 1.00 kg of water. What is the molality? Calculate the molality of 1 L of 1.22M sucrose (MW 342.3 g/mol) solution. The density of the solution is 1.12 g/mL.

  8. C. Mole Fraction • Amount of the component we are interested divided by the total amount of all the components of the mixture Mole fraction of A (XA) =

  9. example • Consider a solution that has 1.00 mole ethanol (46.1 g) dissolved in 9.00 moles of water (162 g). Find the mole fraction of each.

  10. D. Mass Percent • Common unit of measurement for consumer products • Mass of one component divided by the total mass of the mixture multiplied by 100%

  11. Example • Consider an alcohol-water mixture that is prepared by dissolving 46.1 g of ethanol in 116.2 g of water. What is the mass percent of ethanol in the mixture?

  12. IV. Pressure, Temperature and Solubility • Solid solubility and Temperature • When a solid is dissolved an equilibrium is established • An increase in temp will shift the equilibrium

  13. B. Gas Solubility • Solubility of gasses decreases with increasing temperature • Why do fish go to bottom of lake in the summer? • Gases that dissolve to an appreciable extent in water do so in an exothermic process • So what do you need to do to get the gas out of the water?

  14. V. Pressure and Solubility A. Henry’s Law • Solubility of a gas in a liquid is directly proportional to the gas pressure • Henry’s Law • cg = kHPg • cg = molar concentration (mol/L) • kH = Henry’s law constant (mol/L · atm) • Pg =pressure of a gaseous solute (atm)

  15. Example Deep-sea diving • Solubility of gases is the concentration of gases in equilibrium with substances in gaseous state • What would happen if we increased the pressure?

  16. VI. Colligative Properties • Properties of a solution that depend only in the number of solute particles per solvent molecule and not the nature of the solute or solvent

  17. A. Raoult’s Law • Equilibrium vapor pressure: pressure of the vapor when the liquid and the vapor are in equilibrium

  18. Example • Solution contains 82.0 g of ethylene glycol in 100g of water at 22oC. (where is vapor pressure of water is 19.88 mm Hg) what is the new vapor pressure?

  19. B. Boiling Point Elevation • You have a solution of 0.200 mole of sugar in 100.0 g of benzene • The normal vapor pressure of pure benzene is 400 mmHg, what is the new vapor pressure of this solution?

  20. This vapor pressure lowering causes a BP elevation! • Normal BP is the temp at which vapor pressure =760 mm Hg • BP elevation

  21. B. BP elevation • Leads to general equation: • The melting point of water is 0oC at 1atm. If 12.15 g of ZnSO4 (MW 161.5 g/mol) are dissolved into 161.2 g of water: • What is the molality of the solution • What is the new melting point of the solution?

  22. C. Freezing Point Depression • Freezing point of a solution will be lower than of a pure solvent • General equation • Why add antifreeze to your car?

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