Theory Of Dilute Solution


Cover page i
Dedication ii
Acknowledgement iii
Tables of Content iv
Table of figures vi
1.1 Introduction 1
1.2 History of colligative property 3
1.3 Abnormal molecular mass 4

CHAPTER TWO: Lowering of Vapour pressure
2.1 Vapour pressure 5
2.2 Raoult’s Law 8
2.3 Ideal solutions and deviations from Raoult’s law 10
2.4 Properties of real solutions 11
2.5 Measurement of the lowering of vapour pressure 11
2.5.1 The Barometric method 12
2.5.2 The manometric method 12
2.5.3 The Ostwald and Walker’s dynamic method 13

CHAPTER THREE: The elevation of boiling point
3.1 Introduction to boiling point elevation 15
3.2 Relationship between the elevation of boiling point and lowering of vapour pressure 16
3.3 The general equation for calculations at dilute concentration 18
3.4 Ebullioscopic constants for some compounds 19
3.5 Measurement of boiling point elevation 20
3.5.1 The Landsberger-walker method 20
3.5.2 The cottrell’s method 21
3.6 Uses of boiling point elevation 23

CHAPTER FOUR: Freezing point depression
4.1 Introduction to freezing point depression 24
4.2 Relationship between depression of freezing point and lowering of vapour pressure 25
4.3 Measurement of freezing point depression 26
4.3.1 The Beckmann’s method 27
4.3.2 The Rast’s camphor method 28
4.4 Uses of freezing point depression 30
5.1 Osmosis 32
5.2 History of osmotic pressure 33
5.3 What is osmotic pressure 33
5.4 Applications of osmotic pressure 35
5.5 Conclusion 37
References 38

Fig 1: Lowering of vapour pressure by a non-volatile solute.
Fig 2: Negative and positive deviation
Fig 3: Measurements of vapour pressure of aqueous solutions with a manometer
Fig 4: Ostwald-Walker method of measuring the relative lowering of vapour pressure
Fig 5: A graph of vapour pressure against temperature
Fig 6: Landberger-Walker method
Fig 7: Beckmann thermometer reading to 0.01K
Fig 8: Cottrell’s Apparatus
Fig 9: Relationship between lowering of vapour pressure and depression of freezing point
Fig 10: Relation between lowering of vapour pressure and depression of freezing point
Fig 11: Determination of depression of melting point by capillary method
Fig 12: Determination of depression of melting point by electrical method
Fig 13: The equilibrium involved in the calculation of osmotic pressure.
Fig 14: A simple version of the osmotic pressure experiment


The knowledge of the laws of solutions has been said, to be important because almost all the chemical processes which occur in nature, whether in animal or vegetable organisms, or in the non-living surface of the earth, and also those which are carried out in the laboratory, take place between substances in solution. For example, a sound judgment regarding physiological processes is impossible without this knowledge; and this holds true for the greater number of the scientifically and technically important reactions. Solutions are more important than gases, for the latter seldom react together at ordinary temperatures, whereas solutions present the best conditions for the occurrence of all chemical processes (Homer, 1980).

A dilute solution has a low concentration of the solute compared to the solvent. The opposite of a dilute solution is a concentrated solution, which has high levels of solute in the mixture.

Dilute solutions containing non-volatile solute exhibit the following properties:
(1) Lowering of the Vapour Pressure
(2) Elevation of the Boiling Point
(3) Depression of the Freezing Point
(4) Osmotic Pressure

The essential feature of these properties is that they depend only on the number of solute particles present in solution. Being closely related to each other through a common explanation, these have been grouped together under the class name Colligative Properties (Greek colligatus = Collected together) (Bahl, et al., 2012).

Physical properties can be divided into two categories. Extensive properties (such as mass and volume) depend on the size of the sample. Intensive properties (such as density and concentration) are characteristic properties of the substance; they do not depend on the size of the sample being studied. This section introduces a third category that is a subset of the intensive properties of a system. This third category, known as colligative properties, can only be applied to solutions. By definition, one of the properties of a solution is a colligative property if it depends only on the ratio of the number of particles of solute and solvent in the solution, not the identity of the solute.

A colligative property may be defined as one which depends on the number of particles in solution and not in any way on the size or chemical nature of the particles. In other words, colligative properties are a set of solution properties that can be reasonably approximated by assuming that the solution is ideal.
Here we consider only properties which result from the dissolution of nonvolatile solute in a volatile liquid solvent. They are essentially solvent properties which are changed by the presence of the solute. The solute particles displace some solvent molecules in the liquid phase and therefore reduce the concentration of solvent, so that the colligative properties are independent of the nature of the solute.
For a given solute-solvent mass ratio, all colligative properties are inversely proportional to solute molar mass.

Measurement of colligative properties for a dilute solution of a non-ionized solute such as urea or glucose in water or another solvent can lead to determinations of relative molar masses, both for small molecules and for polymers which cannot be studied by other means. Alternatively, measurements for ionized solutes can lead to an estimation of the percentage of dissociation taking place.
Colligative properties are mostly studied for dilute solutions, whose behavior may often be approximated as that of an ideal solution.