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:
 PV = n RT
 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 isoosmotic 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