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Chapter 13

Chapter 13. Properties of solutions. Quantitative terms: Concentrated, dilute Qualitative expressions: Molarity (M); Units: moles / L Mass percent (weight percent) Mass of solute x 100 Mass of solution Mole fraction ( ); Mixture of A, B and C:  A = n A / (n A + n B + n C )

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Chapter 13

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  1. Chapter 13 Properties of solutions

  2. Quantitative terms: Concentrated, dilute Qualitative expressions: Molarity (M); Units: moles / L Mass percent (weight percent) Mass of solute x 100 Mass of solution Mole fraction (); Mixture of A, B and C: A = nA / (nA + nB + nC) Molality (m); Units: moles of solute/kg of solvent Solution composition

  3. 10.0g of ethanol (C2H5OH), mixed with 100.0g of water, to give a final volume of 110 mL. Calculate: (i) Molarity, (ii) mass percent, (iii) mole fraction and (iv) molality of ethanol in solution. Composition calculations

  4. ppm • A solution of water contains 0.00023g of SO2 per L of solution. Concentration of SO2 in ppm? • ppm of component = mass of component in soln x 106 total mass of component

  5. Sweet’n’Low vs. Equal in water Factors affecting solubility? Assume 3 distinct steps for solution formation: Expanding the solute (H1) - endothermic Expanding the solvent (H2) - endothermic Interaction of solute and solvent (H3) - exothermic Hsoln = H1 + H2 + H3 Oil and water NaCl and water Energies in solution formation

  6. Correlation between molecular structure, polarity and solubility. Example: Consider some important vitamins A, D, E and K: Fat soluble (mainly non-polar) B and C: Water soluble (mainly polar) Hydrophobic; Hydrophilic A, D, E and K stored in fatty tissue. Excessive amounts: Vitaminosis B and C; excreted from the body; replace regularly. Scurvy (Vitamin C deficiency); Limeys Structure effects on solubility

  7. Soda – bottled under high pressure of CO2 (high conc. of dissolved CO2 in liquid) Why does soda ‘fizz’ when opened? Relationship between gas pressure and conc. of gas dissolved gas: Henry’s Law; C=kP (William Henry – 1801) C = Concentration k = Henry’s Law Constant (different for each solute/solvent pair) P = Partial pressure Amount of gas dissolved in a solution  pressure of gas above the solution Pressure Effects

  8. Pressure Effects

  9. Bottle of ‘Jolt’ cola at 25 °C, has CO2 gas present at 7.5 atm above the solution. Assuming that PCO2 in atmosphere is 4.0 x 10-4 atm, calculate equilibrium concentrations of CO2 in cola both before and after bottle is opened. Henry’s Law Calculation

  10. Solids in water – Gases in water – Temperature Effects (aqueous solns)

  11. Pressure of gas (vapor) above solvent/solution (higher for more volatile solvents) Liquid solutions – different properties to pure liquids Antifreeze (ethylene glycol) Salt on icy roads Solutes change properties of pure solvent (H2O) Nonvolatile solute reduces vapor pressure of a solvent. Vapor Pressure

  12. Vapor pressures of solutions (François Raoult ~ 1870) Raoult’s Law; Psoln = solvent . P°solvent Obeying Raoult’s Law: Plot of Psoln vs solvent = straight line Vapor Pressure of a solution  mole fraction of solvent present. Vapor Pressure

  13. Calculate the expected VP at 25 °C for a solution prepared by dissolving 250.0 g sucrose (MW 342.3 g/mol) in 750 mL of water. At 25 °C, density of water is 0.9971 g/mL, vapor pressure is 23.76 torr. Calculating VP of a solution

  14. NaCl (s)  Na+(aq) + Cl- (aq) 1 mole 2 moles Na2SO4 2Na+ (aq) + SO42-(aq) 1 mole 3 moles Predict VP of a solution prepared by mixing 50.0g solid Na2SO4 (MW 142 g/mol) with 200g of water @ 25 °C. VP of pure water @ 25 °C is 23.76 torr. VP calculations involving ionic solutes

  15. A property of a solution that depends only on the number of solute particles present, not on their chemical identity. Look at several of these: Boiling point elevation Freezing point depression Osmotic Pressure (and dialysis) Colligative properties

  16. Impurities lower freezing points and raise boiling points of liquids T = Kbmsolute T = Kfmsolute Sugar in water to make candy antifreeze in radiator salt on icy roads Freezing point depression

  17. What mass of ethylene glycol (C2H6O2), the main component of antifreeze, must be added to 10.0L of water to produce a solution for use in a car’s radiator that freezes at –23.3 °C? Density of water = 1g/mL. Kf (H2O) = 1.86 °C.kg/mol Useful method for determining molecular weight: A sample of a human hormone weighing 0.546g was dissolved in 15.0g benzene. The fpd was determined to be 0.240 C. What’s the molecular weight of the hormone? Freezing point depression

  18. Determining molecular weight • A sample of a human hormone weighing 0.546g was dissolved in 15.0g benzene. The fpd was determined to be 0.240 C. What’s the molecular weight of the hormone? Kf(benzene) = 5.20 °C.kg/mol

  19. Osmosis • Semipermeable membrane: permits passage of some components of a solution (cell membranes) • Osmosis: the movement of a solvent from low solute concentration to high solute concentration. • There is movement in both directions across a semipermeable membrane. • As solvent moves across the membrane, the fluid levels in the arms becomes uneven.

  20. Osmosis • Eventually, pressure differences stop the levels in the arms from moving further apart.

  21. Osmotic Pressure () • Osmotic pressure, , is the pressure required to stop osmosis:  = osmotic pressure M = Molarity (mol/L) R = Ideal Gas Constant T = Temperature (K)

  22. Molecular weight determination • To determine the MW of a particular protein, 1.00x10-3 g of it was dissolved in enough water to make 1.00 L of solution.  of this solution found to be 1.12 torr at 25 °C. MW of protein?

  23. Reverse Osmosis

  24. Reverse Osmosis – Desalination Plants

  25. Use of ‘artificial kidneys’ to clean the blood of its waste products when kidneys inoperative. Kidneys work by allowing small waste particles to be expelled, whilst retaining larger protein molecules. Dialysis machines work in exactly the same way Dialysis

  26. Left cylinder contains concentrated NaCl solution. Right cylinder contains pure water. What’s going on in each case? Left carrot has shrunk. Right carrot has expanded. Osmosis and carrots?

  27. Why is osmosis important to human beings? Analogous to situation with carrots we just saw. Red blood cells have a certain osmotic pressure; their surrounding medium (plasma) needs to have the same osmotic pressure, or nasty things happen….. Solutions with equal osmotic pressures known as isotonic. Thus, contents of red blood cells and surrounding plasma are isotonic. When somebody needs intravenous injections, some solution is needed. This is routinely 0.89% sodium chloride in water, or saline solution. This is isotonic with red blood cells, thus safe. But…. Osmosis and biological organisms

  28. What if you were on a drip that contained only distilled water? Example of a Hypotonic solution. Lower osmotic pressure that red blood cells…. What would happen to the red blood cells? Water would flow into them, causing them to burst. Condition known as hemolysis. Very serious. Analogous to the expanded carrot. Hemolysis

  29. What if you were on a drip that contained a solution of NaCl with a larger concentration that 0.89%? Hypertonic solution. Higher osmotic pressure than in red blood cells. What would happen here? Water would diffuse out of red blood cells. Red blood cells would shrivel up. Equally serious condition – Crenation. Execution by lethal injection uses (amongst other things) a lethal dose of KCl solution. Crenation

  30. Similar to solutions, except that solute particle sizes are much larger. Solutions: solute particles  1nm diameter (1nm = 1x10-9 m); very small – solutions usually clear. (Sea water) Colloids: solute particles from ~1 to 1000nm diameter. Colloids usually opaque, milky. (Milk) Dividing line between solutions and heterogeneous mixtures Colloids (colloidal systems)

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