Colligative Properties Lecture Notes PDF

Colligative properties Dr. Ibrahim Siddig Outlines What are the colligative properties Colligative properties of solutions Types of colligative properties Boiling point elevation Freezing point depression Osmotic pressure and osmosis Colligative properties When a solute is...

Colligative properties Dr. Ibrahim Siddig Outlines What are the colligative properties Colligative properties of solutions Types of colligative properties Boiling point elevation Freezing point depression Osmotic pressure and osmosis Colligative properties When a solute is dissolved in a solvent, the properties of the solvent are changed by the presence of the solute The magnitude of the change generally is proportional to the amount of solute added Some properties of the solvent are changed only by the number of solute particles present, without regard to the nature of the solute Such properties are called Colligative properties of the solution Colligative properties of solutions When a nonvolatile solute is dissolved in a solvent, certain properties of the resulting solution are largely independent of the nature of solute and are determined by the concentration of the solute particles The presence of solute particles changes the physical properties of the solution Depends on the NUMBER of solute particles Types of colligative properties Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles: 1. Vapour pressure lowering 2. Boiling point elevation 3. Melting point depression 4. Osmotic pressure Ionization of solutes Ionic compounds dissolve in water and separate into positive and negative ions, such solutes are termed electrolytes and their ionization in water has several consequences that are often important in pharmaceutical practice. These ions allow the solution to conduct electricity Non-electrolytes sometime dissolve but they don’t form ions. Therefore, the solution will not conduct electricity Electrolyte vs Nonelectrolyte Solutions The colligative properties of solutions of different non-electrolytes is identical for identical molal concentrations The reason is that they are nonelectrolytes and are thus not ionized when introduced into water and each molecule keeps its identity and remains as an entity. On the other hand, electrolytes will ionize, and each ion will act as an entity. These ions allow the solution to conduct electricity Van Hoff's factor The number of ionic entities is called van Hoff's factor (i) The colligative properties of freezing point and boiling point are proportional to the number of particles present in the solution. Van Hoff's factor, i NaCl → i= 1+1= 2 moles CaCl2 → i = 1+ 2 = 3 moles Al2(SO4)3 → i = 2+ 3 = 5 moles Molality The molality of a solution is the number of moles of solute divided by the weight of the solvent in kilograms. molality (M) = moles solute/weight of solvent kg Vapor pressure depression Go back to lec 4 (slides 22-31) Less surface area, less evaporation Boiling point elevation The boiling point of a liquid is the temperature at which the vapor pressure of the liquid is equal to the external pressure of 760 mm Hg. Since the vapor pressure of a solvent is lowered when a nonvolatile solute is added, therefore the solution must be heated to a higher temperature than the pure solvent to reach the same 1 atmospheric pressure. The boiling point of a solution is thus elevated in comparison to the boiling point of the pure solvent Boiling point elevation Tb= iKbm where ∆Tb is the elevation of the boiling point, m is the molality of the solution and Kb is the proportionality constant, which can be also defined as the boiling point elevation for one molal (m) diluted solute solution i.e., Kb = ∆Tb/m Freezing Point Depression Solutes lower the freezing point of many solutions. That means that the solution will freeze at a lower temperature than normal. The freezing point depression is the difference between the solution’s freezing point and the normal freezing point of the solvent. The relation between the freezing point depression and the molality is given as: ΔTf = iKf m Here Kf is the molal freezing point depression constant of the solvent Example Calculate the Freezing Point of a solution of 90 grams of glucose (MW = 180) a non-electrolyte, dissolved in 750 grams of water. The Kf for water = 1.86 C/m. The Freezing Point of pure water = 0 C  Solution Moles of glucose=90/180=0.5moles Weight of water= 750 / 1000 = 0.750 Kg Molality=0.5/0.75= 0.667 moles  Tf = (1.86) (1) (0.667) = 1.24 Tf =0 -1.24 C = -1.24 C Values for Kb and Kf for some solvents Example A solution of 5.00 g lauryl alcohol (solute) dissolved in 100 g benzene (solvent) freezes at 4.1oC. Calculate molar mass of lauryl alcohol given that the Fp = 5.5° for benzene  ΔTf=5.5-4.1=1.4oC ΔTf = 1.4oC Use ΔTf= Kfm; i=1, = ΔTf/iKf = 1.4oC/(5.12oC/m) = 0.27 m molality is mol solute/kg solvent=(0.5/Mw)/0.1=0.27mol Molality=(wt/Mwt)/0.1 Molality=(5/Mwt)/0.1 0.27x0.1=5/Mwt Mwt of the solute=5/0.027=185g Osmotic pressure and osmosis The osmotic pressure of a solution is the external pressure that must be applied to the solution in order to prevent it being diluted by the entry of solvent via a process known as osmosis. Osmosis and Osmotic pressure The flow of solvent molecules through a semipermeable membrane from the less concentrated side (less solute) to the more concentrated side (more solute). A semipermeable (porous) membrane allows certain molecules to go through it, in part due to their small size (ex. water molecules). Osmotic pressure: pressure needed to prevent osmosis Osmolarity and Iso-osmotic solutions The term osmolarity is used to describe the number of solute particles in a volume of fluid ( is the number of active particles per liter of solution) Iso-osmotic solutions: If two solutions are separated by a perfect semipermeable membrane, i.e., a membrane which is permeable only to solvent molecules, and no net movement of solvent occurs across the membrane, then the solutions are said to be iso-osmotic and will have equal osmotic pressures Isotonic solutions Biological membranes do not always function as perfect semipermeable membranes and some solute molecules as well as water are able to pass through them. If two iso-osmotic solutions remain in osmotic equilibrium when separated by a biological membrane, they may be isotonic with respect to that membrane. Adjustment of isotonicity is particularly important for formulations intended for parenteral routes of administration Clinical importance of osmotic pressure It is important, to make sure that the osmotic pressure of solutions for injection is about the same as that in the blood. Such solutions are called isotonic solutions. Solutions with a higher osmotic pressure are hypertonic and those of a lower osmotic pressure are said to be hypotonic. Also, to reduce or avoid discomfort on administration of solutions to the delicate membranes (e.g., ophthalmic, nasal, vaginal solutions) it is very important to ensure that the solutions are isotonic with the tissues. Osmosis in blood cells If the solute concentration outside the cell is greater than that inside the cell, the solution is hypertonic. Water will flow out of the cell, and crenation results. Osmosis in blood cells If the solute concentration outside the cell is less than that inside the cell, the solution is hypotonic. Water will flow into the cell, and hemolysis results. ✓Osmosis is important in many biological processes. ✓A cell might be thought of as an aqueous solution surrounded by a semipermeable membrane. ✓The solutions surrounding cells must have the same osmotic pressure. Otherwise, water will either leave the cell, dehydrating it, or enter the cell, causing it to burst. Osmotic pressure  =iMRT = osmotic pressure i=van't Hoff factor M= molarity of solute particles T= absolute temperature R =universal gas constant Solve this problem A 20.0 mg sample of protein is dissolved in water to make 25.0 mL of solution. The osmotic pressure of the solution is 0.56 torr at 25.0 oC. What is the molar mass of the protein? Osmotic pressure equation: p = MRT In this problem, p = 0.56 torr, R = constant, T = 25 oC and M = ?