Colligative Properties & Molecular Weight Determination HAWLER MEDICAL UNIVERSITY COLLEGE OF PHARMACY PHARMACEUTICS DEPARTMENT Dec. 2015 Assist. Lecturer: Hewa Abdullah

Outline Vapor pressure lowering Boiling point elevation Freezing point depression Osmotic pressure Molecular weight determination

Colligative properties When a nonvolatile solute is combined with a volatile solvent, the vapor above the solution is provided solely by the solvent. The vapor pressure of a solution containing a nonvolatile solute is lowered. The freezing point, boiling point, and osmotic pressure of a solution also depend on the relative proportion of the molecules of the solute and the solvent.

Lowering of the Vapor Pressure X1 + X2= 1 X1 is mole fraction of the solvent X2 is mole fraction of the solute Moles of X= weight (g)/ molecular weight

∆P is the lowering of the vapor pressure P1˚ is the vapor pressure of pure solvent X2 is mole fraction of solute N1 mole fraction of solvent N2 mole fraction of solute ∆P / P1˚ is the relative vapor pressure lowering

Example/Calculate the relative vapor pressure lowering at 20° C for a solution containing 171.2 g of sucrose (W2) in 1000 g (W1) of water. The molecular weight of sucrose (M2) is 342.3 and the molecular weight of water (M1) is 18.02g/mole.

Determination of the Vapor Pressure of Solutions The vapor pressure of a solution may be determined directly by means of a manometer. The isopiestie method is used frequently

Vapor pressure osmometer This instrument has been applied to monitoring diuretic therapy, quantitating sodium in isotonic solutions, and studying the colligative properties of parenteral solutions.

Elevation of the Boiling Point The boiling point is defined as the temperature at which the vapor pressure of a liquid equals the atmospheric pressure (760 mmHg). The boiling point of a solution of a nonvolatile solute is higher than that of the pure solvent. The boiling point of pure water is 100°C, but that boiling point can be elevated by the adding of a solute such as a salt Boiling point elevation depends only on the mole fraction of the solute

Determination of Boiling Point Elevation The boiling point elevation ∆Tb is a colligative property of the solution, and for dilute solutions is found to be proportional to the molal concentration of the solution: ∆Tb= Kb * m ∆Tb is the boiling point elevation Kb is called the molal elevation constant or the ebullioscopic constant

Solutions may be produced for the purpose of raising the boiling point and lowering the freezing point, as in the use of ethylene glycol (antifreeze) in automobile cooling systems. The ethylene glycol (antifreeze) protects against freezing by lowering the freezing point and permits a higher operating temperature by raising the boiling point.

Example / A 0.200 m aqueous solution of a drug gave a boiling point elevation of 0.103° C. Calculate the approximate molal elevation constant for the solvent? ∆Tb= Kb * m

Freezing point depression The freezing point is depressed due to the vapor pressure lowering phenomenon. Freezing point : is defined as the temperature at which the solid and the liquid phases are in equilibrium under a pressure of 1 atm. The freezing point of pure water is 0°C, but that melting point can be depressed by the adding of a solute such as a salt.

The use of ordinary salt (sodium chloride, NaCl) on icy roads in the winter helps to melt the ice from the roads by lowering the melting point of the ice. A solution typically has a measurably lower melting point than the pure solvent. A 10% salt solution was said to lower the melting point to -6°C and a 20% salt solution was said to lower it to -16°C.

Freezing point for pure solvent and solution

Determination of freezing point lowering The freezing point depression of a solvent is a function only of the number of particles in the solution ∆Tf is the freezing point depression Kf is the molal depression constant or the cryoscopic constant

Practical Applications of Colligative Properties 1.Preparation of isotonic intravenous and isotonic lachrymal solutions. 2.Determination of the molecular weight of solutes or in the case of electrolytes, the extent of ionization. 3.They also may be used in experimental physiology as in immersion of tissues in salt solutions which are isotonic with the tissue fluids to prevent changes or injuries of the tissues.

Osmotic pressure Solvent passes into more conc. solution followed by increasing its volume The passage of the solvent can be prevented by application of a pressure The pressure to prevent transport is the osmotic pressure

1- The addition of a nonvolatile solute to the solvent forms a solution in which the vapor pressure of the solvent is reduced. 2- If pure solvent is now placed adjacent to the solution but separated from it by a semipermeable membrane, solvent molecules will pass through the membrane into the solution in an attempt to dilute out the solute and raise the vapor pressure back to its original value

3- The osmotic pressure may be determined either by measuring the hydrostatic head appearing in the solution or by applying a known pressure that just balances the osmotic pressure and prevents any net movement of solvent molecules into the solution

Measurement of Osmotic Pressure The relationship between osmotic pressure and the concentration of a non-electrolyte is given for dilute solutions, which may be assumed to exhibit ideal behavior, by the van't Hoff equation: P V = n R T where V is the volume of solution, n, is the number of moles of solute, T is the absolute temperature and R is the gas constant. The osmotic pressure of solutions of different nonelectrolytes is proportional to the number of molecules in each solution.

The osmotic pressures of two nonelectrolyte solutions of same molal concentration are identical. For example, a solution containing 34.2g of sucrose (mol wt. 342) in 1000 g of water has the same osmotic pressure as a solution containing 18.0 g of anhydrous dextrose (mol wt. 180) in 1000 g water. These solutions are said to be iso- osmotic with each other because they have identical osmotic pressures.

Colligative properties for nonvolatile solutes Vapor pressure is always lower Boiling point is always higher Freezing point is always lower Osmotic pressure drives solvent from lower concentration to higher concentration

MOLECULAR WEIGHT DETERMINATION The four colligative properties; vapor pressure lowering, freezing point lowering, boiling point elevation, and osmotic pressure may be used to calculate the molecular weights of nonelectrolytes present as solutes. The lowering of the vapor pressure of a solution containing a nonvolatile solute depends only on the mole fraction of the solute.

Choice of colligative properties Each of the colligative properties seems to have certain advantages and disadvantages for the determination of molecular weights. The boiling point method can be used only when the solute is nonvolatile and when the substance is not decomposed at boiling temperatures.

The freezing point method is satisfactory for solutions containing volatile solutes, such as alcohol, since the freezing point of a solution depends on the vapor pressure of the solvent alone. The freezing point method is easily executed and yields results of high accuracy for solutions of small molecules.

Osmotic pressure measurements do not have this disadvantage. The cryoscopic and newer vapor pressure techniques are the methods of choice, except for high polymers, in which instance the osmotic pressure method is used