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3- Solutions & It's Colligative Properties(Physical Pharmacy)

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Solutions & It's Colligative Properties

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3- Solutions & It's Colligative Properties(Physical Pharmacy)

  1. 1. Khalid T Maaroof MSc. Pharmaceutical sciences School of pharmacy – Pharmaceutics department 1 Online access: bit.ly/physicalpharmacy Solutions Physical Pharmacy 10/31/2015
  2. 2. Solutions 2
  3. 3. Types ofsolutes Na+ Cl- • Electrolytes (Conductive): Dissociation in to ions in solution Eg: NaCl • Nonelectrolytes (no conductivity): no dissociation Eg: suger
  4. 4. Concentration expressions for solutions 4  Molarity?  Normality?  Molarity?  Mole fracction?  Mole percent?  Percent  Percent by weight % w/w  Percent by volume %v/v  Percent weight in volume % w/v
  5. 5. Concentration expressed as percentage 5 – Percent weight-in-weight (w/w) is the grams of solute in 100 grams of the solution. – Percent weight-in-volume (w/v) is the grams of solute in 100ml of the solution. – Percent volume-in-volume (v/v) is the milliliters of solute in 100ml of the solution. A 20 % w/w solution contains 20g solute how many grams the solvent is?
  6. 6. Molarity, Normality, and Molality 6  Molarity and normality both depend on the volume of the solvent, so their values are affected by change of volume caused by factors such as change in temperature.  Molality doesn’t has this disadvantage.
  7. 7. Mole fraction 7  In a solution containing 0.01 mole of solute and 0.04 mole of solvent, the mole fraction of the solute is 0.2 and for solvent it is 0.8 Mole percent = mole fraction X 100
  8. 8. 8  An aqueous solution of ferrous sulfate was prepared by adding 41.50 g of FeSO4 to enough water to make 1000 mL of solution. The density of the solution is 1.0375 and the molecular weight of FeSO4 is 151.9. Calculate  (a) molarity  (b) molality  (c) mole fraction of FeSO4, mole fraction of water, and the mole percent of the two constituents  (d) % w/w of FeSO4.
  9. 9. Ideal and real solutions 9  Ideal solution is defined as a solution in which there is no change in the properties of the components other than dilution when they are mixed to form the solution  Molecules exhibit complete freedom of motion and randomness of distribution in the solution.  Ideality in solutions means complete uniformity of attractive forces
  10. 10. Ideal Solutions and 10  P = pA + pB  pA = pA◦ XA  pB = pB◦ XB  What is the partial vapor pressure of benzene and of ethylene chloride in a solution at a mole fraction of benzene of 0.6? The vapor pressure of pure benzene at 50◦C is 268 mm, and the corresponding pA ◦ for ethylene chloride is 236 mm. Raoult's Law States that, in an ideal solution, the partial vapor pressure of each volatile constituent is equal to the vapor pressure of the pure constituent multiplied by its mole fraction in the solution. Thus, for two constituents A and B,
  11. 11. 11 Vapor pressure composition curve (for previous example)
  12. 12. Real Solutions 12  Ideality in solutions presupposes complete uniformity of attractive forces.  Many examples of solution pairs are known, however, in which the “cohesive” attraction of A for A exceeds the “adhesive” attraction existing between A and B.  Similarly, the attractive forces between A and B may be greater than those between A and A or B and B.  Such mixtures are real or nonideal; that is, they do not adhere to Raoult’s law  Two types of deviation from Raoult’s law are recognized, negative deviation and positive
  13. 13. 13 When the “adhesive” attractions between molecules of different species exceed the “cohesive” attractions between like molecules, the vapor pressure of the solution is less than that expected from Raoult’s ideal solution law, and negative deviation occurs. Negative deviation Adhesion > Cohesion
  14. 14. 14 When the “adhesive” attractions between molecules of different species are weaker than “cohesive” attractions between like molecules, the vapor pressure of the solution is more than that expected from Raoult’s ideal solution law, and positive deviation occurs. Positive deviation Adhesion < Cohesion
  15. 15. Questions ! 10/31/201515
  16. 16. Colligative properties 16
  17. 17. Colligative properties 17  Colligative properties of solutions are those that affected (changed) by the presence of solute and depend solely on the number (amount of solute in the solutions) rather than nature of constituents.  Examples of colligative properties are:  Vapor pressure  Boiling point  Freezing point  Osmotic pressure
  18. 18. Colligative vs Non-colligative 18 Compare 1.0 M aqueous sugar solution to a 0.5 M solution of salt (NaCl) in water. both solutions have the same number of dissolved particles any difference in the properties of those two solutions is due to a non-colligative property. Both have the same freezing point, boiling point, vapor pressure, and osmotic pressure
  19. 19. Non-Colligative Properties Sugar solution is sweet and salt solution is salty. Therefore, the taste of the solution is not a colligative property. Another non-colligative property is the color of a solution. Other non-colligative properties include viscosity, surface tension, and solubility. 19
  20. 20. Lowering of vapor pressure Vapor pressure: Pure solvent > solutions
  21. 21. Lowering of vapor pressure 21  According to raoult’s law Psolvent = Pºsolvent Xsolvent  But if the solute used in non volatile only pressure from solvent can be considered.  So:  On the other hand Psolute = Pºsolute Xsolute Psolution = Pºsolvent Xsolvent X1 = mole fraction of solvent X2 = mole fraction of solute
  22. 22. 22  ∆p = p1◦ − p is the lowering of the vapor pressure and ∆p/p1◦ is the relative vapor pressure lowering.  The relative vapor pressure lowering depends only on the mole fraction of the solute, X2, that is, on the number of solute particles in a definite volume of solution. Therefore, the relative vapor pressure lowering is a colligative property.
  23. 23. 23  Calculate the relative vapor pressure lowering at 20◦C for a solution containing 171.2 g of sucrose (w2) in 100 g (w1) of water. The molecular weight of sucrose (M2) is 342.3 and the molecular weight of water (M1) is 18.02 g/mole.
  24. 24. Boiling point elevation 24 Boiling point elevation is a colligative property related to vapor pressure lowering. The boiling point is defined as the temperature at which the vapor pressure of a liquid equals the atmospheric pressure. Due to vapor pressure lowering, a solution will require a higher temperature to reach its boiling point than the pure solvent.
  25. 25. Elevation of the Boiling Point 25  The boiling point of a solution of a nonvolatile solute is higher than that of the pure solvent owing to the fact that the solute lowers the vapor pressure of the solvent. ΔTb = K X2 ΔTb = Kbm boiling point is a colligative property
  26. 26. 26  In dilute solutions: ΔTb = K X2 ΔTb = Kbm Tb: is known as the boiling point elevation Kb: is called the molal elevation constant. m: is molality of solvent
  27. 27. Freezing Point 27 Every liquid has a freezing point - the temperature at which a liquid undergoes a phase change from liquid to solid. When solutes are added to a liquid, forming a solution, the solute molecules disrupt the formation of crystals of the solvent. That disruption in the freezing process results in a depression of the freezing point for the solution relative to the pure solvent.
  28. 28. Depression of the Freezing Point 28 ∆T f = Tº f – T f Kf is the molal epression constant
  29. 29. 29 What happens to the triple point?
  30. 30. 30
  31. 31. Osmotic Pressure 31 When a solution is separated from a volume of pure solvent by a semi-permeable membrane that allows only the passage of solvent molecules, the height of the solution begins to rise. The value of the height difference between the two compartments reflects a property called the osmotic pressure of a solution.
  32. 32. Osmotic Pressure 32 Where π is the osmotic pressure . V is the volume of the solution in liters. n is the number of moles of solute. R is the gas constant, equal to 0.082 liter atm/mole deg. T is the absolute temperature.  Van't Hoff and Morse Equations for Osmotic Pressure:
  33. 33. 33
  34. 34. MOLECULAR WEIGHT DETERMINATION 34  The four colligative properties can be used to calculate the molecular weights of nonelectrolytes present as solutes. Using vapor pressure lowering Using boining point elevation
  35. 35. 35 Using Freezing point depression M2 = 𝑔 𝑅𝑇 Π Using Osmotic pressure
  36. 36. 36
  37. 37. 37
  38. 38. 38
  39. 39. Questions ! 10/31/201539

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