Boiling point, molecular elevation of

Corollary.—The molecular elevation of boiling-point is independent of the nature of the solute. 

Hence for i mole dissolved mice the molecular elevation of the boiling point of water is 514 8° For 1 mole dissolved in 1 liter of water the rise of boiling point is o 5148° It should be pointed out that the expressions for molecular depression of freezmg point and molecular elevation of boiling point are never realised in practice,... 

The molecular elevation of boiling point is the elevation of the boiling point produced when one gram-molecular weight of the solute is dissolved in 100 gm. of solvent. 

Raoult s law When a solute is dissolved in a solvent, the vapour pressure of the latter is lowered proportionally to the mole fraction of solute present. Since the lowering of vapour pressure causes an elevation of the boiling point and a depression of the freezing point, Raoult s law also applies and leads to the conclusion that the elevation of boiling point or depression of freezing point is proportional to the weight of the solute and inversely proportional to its molecular weight. Raoult s law is strictly only applicable to ideal solutions since it assumes that there is no chemical interaction between the solute and solvent molecules. 

Where M is the molecular weight of the solute, Dt is the elevation of boiling point in °C, c is the concentration of solute in grams for lOOOgm of solvent, and K is the Ebullioscopic Constant (molecular elevation of the boiling point) for the solvent. K is a fixed property (constant) for the particular solvent. This has been very useful for the determination of the molecular weights of organic substances in solution. 

When electrolyte solutions are subjected to such measurements, abnormal results are obtained. When substances like sodium chloride or magnesium sulphate are examined, the depression of freezing point or the elevation of boiling point is about twice that calculated from the relative molecular mass, with calcium chloride or sodium sulphate these quantities are three times those expected. Keeping in mind what has been said above, we can say that the number of particles in the solution of sodium chloride or magnesium sulphate is twice the number of molecules present, while in the case of calcium chloride or sodium sulphate there are three particles present for each molecule. 

In the 1920s, it was not feasible to accurately measure the molecu-f lar weight of natural or synthetic polymers. Classical methods 1 of molecular weight determina-V tion, those based upon colligative x properties, elevation of boiling point, depression of freezing point and lowering of vapor pressure, worked very well for low-molar-mass compounds, but were essentially useless for macromolecules. Modern instrumental methods that... 

The elevation of boiling point resulting from the addition to the solvent of a known weight of solute, enables the molality of the dissolved substance to be calculated, and hence the molecular weight of the solute if its weight concentration is known. This is the principle of the ebullioscopic method of determining molecular weights. 

Here M is the monomer molecular mass and w the molality of the solution calculated assuming no association or dissociation. Similar equations apply to the elevation of boiling point. 

Ill] Relation between Elevation of Boiling Point and Molecular Weight of Soiute... 

The number average molecular weight is normally determined experimentally by measurements involving the colligative properties of the polymer, e.g. vapour pressure, elevation of boiling point, depression of freezing point, osmotic pressure. 

TABLE 11.9 Molecular Elevation of the Boiling Point (Continued)... 

The constant, K, for the molecular elevation of the boiling-point of carbon disulphide is 23-7.1 The specific heat, C, of liquid carbon disulphide is given by2... 

The use of boiling-point elevation to determine molecular weights is based upon the same type of calculation, using KB instead of Kt. 

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