Vapour pressure colligative

In physical chemistry, we apply the term colligative to those properties that depend upon number of molecules present. The principal colligative properties are boiling point elevation, freezing point depression, vapour pressure lowering, and osmotic pressure. All such methods require extrapolation of experimental data back to infinite dilution. This arises due to the fact that the physical properties of any solute at a reasonable concentration in a solvent are... 

Vapour pressure osmometry is the second experimental technique based on colligative properties with importance for molar mass determination. The vapour pressure of the solvent above a (polymer) solution is determined by the requirement that the chemical potential of the solvent in the vapour and in the liquid phase must be identical. For ideal solutions the change of the vapour pressure p of the solvent due to the presence of the solute with molar volume V/1 is given by... 

The depression of a melting point is one of the simplest manifestations of a colligative property. Other everyday examples include pressure, osmotic pressure, vapour pressure and elevation of boiling point. 

Methods for the determination of Molecular weight based on colligative property are vapour-pressure lowering, boiling point elevation (ebulliometry), freezing-point depression (cryoscopy), and the Osmotic pressure (osmometry). 

The isopiestic method measures a difference in vapour pressure while the isothermal distillation technique depends upon a difference in volume. Despite the specific changes being measured in the techniques each change is proportional to the colligative property of the solution - the lowering of the vapour pressure. 

The vapour pressure osmometer method is more acceptable of all the methods involving measurement of colligative properties because of the sensitivity of the detector. For ideal solvent-solvents with a low heat of vaporisation, the differential thermistors of the VPO can detect differences in temperature of the order of 0.001°C this sensitivity determines the Molecular weight of the samples upto 20,000. 

The up to now most frequently used techniques as, for example, vapour pressure osmometry (VPO) or freezing point depression (with its limitation regarding the solvent dependent measuring temperature) are based upon the colligative properties of the system the classical absolute light-scattering and ultracentrifugation techniques are only occasionally and approximately applicable with respect to the determination of CMC values. Evaluation of critical micelle concentrations which are based on these latter methods suffer considerably from the insensitivity of these techniques if measurements below the CMC, i.e., below about 10-3 mol dm-3, are carried out. More sensitive methods will be discussed below. 

The lowering of the vapour pressure of a pure liquid A by the addition of B (as above) is one example of a colligative property (defined in Frame 51) where such properties are further discussed and include ... 

Real colligative properties are only found in ideal gases and ideal solutions. Examples are osmotic pressure, vapour pressure reduction, boiling-point elevation, freezing-point depression, in other words the osmotic properties. 

Several properties vary in direct proportion to the effective number of osmotically active solute particles per unit mass of solvent and can be used to determine the osmolality of a solution. These colligative properties include freezing point, boiling point and vapour pressure. 

The osmotic pressure of mixtures of solutiom. According to van t Hoif s theory, the osmotic pressure of a solution depends only on the number of the dissolved molecules and not on their nature. Hence the osmotic pressure of a solution containing several substances is equal to the sum of the osmotic pressures due to the substances individually. Thus Dalton s law of partial pressures applies also to solutions. The relative lowering of the vapour pressure, raising of the boiling point, and depression of the freezing point are likewise proportional to the total molal concentration of the solution. Quantities of this kind, which do not depend on the chemical nature, but only on the number of the molecules present, were termed colligative by Ostwald. 

A nonvolatile solute added to a solvent affects not only the magnitude of the vapour pressure above the solvent hut also the freezing point and the boiling point to an extent that is proportional to the relative number of solute molecules present, rather than to the weight concentration of the solute. Properties that are dependent on the number of molecules in solution in this way are referred to as colligative properties, and the most important of such properties from a pharmaceutical viewpoint is the osmotic pressure. 

The experimental measurement of these averages has largely been performed on polymers in solution (Hunt and James, 1999). Since M depends on the measurement of the number of polymer chains present in a given mass, colligative properties such as vapour-pressure depression AP (measured by vapour-phase osmometry) and osmotic pressure (measured by membrane osmometry) relative to the pure solvent, can in principle provide the molar mass through an equation of the form... 

Electrolyte solutions are solutions which can conduct electricity. Colligative properties such as the lowering of the vapour pressure, depression of the freezing point, elevation of the boiling point and osmotic pressure all depend on the number of individual particles present in solution. They thus give information about the number of particles actually present in solution. For some solutes it is found that the number of particles actually present in solution is greater than would be expected from the formula of the compound. 

This is a widely used technique based on the determination of colligative properties. Despite its name, it is not ait osmotic technique at all, but is actually an indirect method of measuring vapour-pressure lowering. The parameter that is measured is the miniscule temperature difference that is obtained in an atmosphere of saturated solvent vapour between droplets of pure solvent and droplets of polymer solution each experiencing solvent evaporation and condensation. This small temperature difference is proportional to the vapour-pressure lowering of the polymer solution at equilibrium hence, it is also proportional to... 

The colligative properties of solutions, e.g. osmotic pressure, boiling point elevations, freezing point depression and vapour pressure reduction, depend on the effect of solute concentration on the solvent aetivity. The ehemieal potential, yt, of a non-electrolyte in dilute solution may be expressed by... 

The importance of this equation is that it can now be related to the experimentally measurable osmotic pressure of a polymer solution (or any other colligative property such as vapour pressure lowering, elevation of boiling point and depression of freezing point) through the chemical potential of the solvent in the solution. 

The determination of single molar mass averages from simple solution properties, such as colligative and viscosity methods, has not been a popular held of research in the past few years. The state of the art has been reviewed by Slade and by Billingham. Since the publication of ref. 2, two major manufacturers (Perkin-Elmer and Hewlett-Packard) have withdrawn from the market so that there is now a much reduced choice in equipment for membrane osmometry and for vapour pressure osmometry. Membrane osmometry as a technique has received virtually no attention during the review period, although its use has been reported in a number of papers. Oman has described studies of the osmotic coefficients of aqueous polyelectrolytes, and Oman and Batho describe theoretical models for... 

Colligative A colligative property of a solute in a dilute solution is an effect whose magnitude depends only on the concentration of the solute particles (number of particles per unit volume of solution) and not on features such as size, shape or chemical composition. Examples are osmotic pressure, depression of freezing point, elevation of boiling point and the vapour pressure of the solution. If the value of any one of these is known for a particular solution, then the values of the others may readily be calculated. 

Introduction of vapour pressures instead of boiling points leads to an alternative way to describe the colligative properties of solutions. At the boiling point Tj p. m), the vapour pressure of water above the solution is equal to that of the external pressure p. This pressure is lower than that of pure water at T=T m) and the difference in these vapour pressures... 

Colligative properties are those properties of a solution of a non-volatile solute which in the limit of infinite dilution, depend only upon the number of solute species present in unit volume of the solution and not upon the nature or size of rtose species. Thus the colligative properties of polymer solutions enable Af to be measured for linear and branched homopolymers and copolymers with equal ease. The four important colligative effects are the osmotic pressure of the solution, the lowering of solvent vapour pressure, the elevation of solvent boiling point and the depression of... 

Those properties which depend only on the number of particles (or molecules) present in the solution and not on the nature of the substance eu"e called colligative properties. In other words, these properties depend on the concentration of the solution. The examples of colligative properties are osmotic pressure, relative lowering of vapour pressure, elevation of boiling point, depression in freezing point etc. 

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