Molecular-weight determination boiling-point elevation

Beckmann thermometer A very sensitive mercury thermometer with a small temperature range which can be changed by transferring mercury between the capillary and a bulb reservoir. Used for accurate temperature measurements in the determination of molecular weights by freezing point depression or boiling point elevation. 

Molecular weight determinations by depression of the freezrng-point are more accurate and far less troublesome than those by the elevation of the boiling-point (p. 440), and the former method should always be preferentially employed if the solubility of the organic compound in the soh ent is sufficiently high. 

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

The vapor pressure of a solvent is lowered on dissolving the solute in it. This lowering for dilute solutions is proportional to die mole fraction of the solute (Raoult s Law). The lowering of the vapor pressure of the solution can be related to the lowering of the freezing point and the elevation of the boiling point. These phenomena serve as a basis for molecular weight determinations. If both components of the solution are volatile, each lowers the vapor pressure of the other and the ratios of the two substances in the liquid and vapor phase are not necessarily the same. Use is made of this fact to separate the two substances by distillation. 

Table 3.6 lists Kf and Kb for several solvents. In general, the higher the molar mass of the solvent, the larger the values of Kf and Kb. If the freezing point depression and boiling point elevation constants are known, the molecular weight of the dissolved solute, M2, can be determined ... 

Molal boiling-point elevation constant a constant characteristic of a particular solvent that gives the change in boiling point as a function of solution molality used in molecular weight determinations. (17.5)... 

A knowledge of the boiling-point elevation and the appropriate constant from Table 1-4 allows one to calculate the molality of the solution, and this figure coupled with the weight of material added gives the molecular weight. The values thus determined are usually accurate to about 10 per cent. 

Boiling point elevation AT = mK[, (the constants have been tabulated) Freezing point depression AT = —mK (the constants have been tabulated) A solution in contact with its pure solvent across a semi-permeable membrane experiences an increase in pressure as pure solvent flows through the membrane into the solution. This osmotic pressure can be measured quite accurately, and through the equation ttV = nRT permits determination of the molecular weight of the solute. 

Fractional Distillation 14-11 Boiling Point Elevation 14-12 Freezing Point Depression 14-13 Determination of Molecular Weight by Freezing Point Depression or Boiling Point Elevation... 

DETERMINATION OE MOLECULAR WEIGHT BY EREEZING POINT DEPRESSION OR BOILING POINT ELEVATION... 

In a similar fashion, solubility measurements (of a gas in a liquid, a liquid in a liquid, or a solid in a liquid) can be used to determine the activity coefficient of a solute in a solvent at saturation. Also, measurements of the solubility of a solid solute in two liquid phases can be used to relate the activity coefficient of the solute in one liquid to a known activity coefficient in another liquid, and freezing-point depression or boiling-point elevation measurements are frequently used to determine the activity of the solvent in a solute-solvent mixture. We have also showed that osmotic-pressure measurements can be used to determine solvent activity coefficients, or to determine the molecular weight of a large polymer or protein. 

Determination of Molecular Weight by Freezing Point Depression or Boiling Point Elevation... 

A similar analysis with L/ replaced by L , the molar heat of vaporization of the pure solvent, relates the boiling-point elevation for nonvolatile solutes to the activity coefficients and the solution molecular weight of the solute. Although more accurate results are obtained by use of freezing-point determination, the boiling-point method is often used when only a small quantity of the solution is available. 

Osmotic pressure. Dissolution of a solute reduces the chemical potential of the solvent and results in a number of observable phenomena known as coUigative properties, such as boiling point elevation, freezing point depression and osmotic pressure. The determination of the extent of change in these properties provides a measure for the molecular weight of the dissolved solute. Thus the molecular weight of a triomacromolecule can be... 

Calculation of the Molecular Formula How do we know the correct molecular formula We can choose the right multiple of the empirical formula if we know the molecular weight. Molecular weights can be determined by methods that relate the freezing-point depression or boiling-point elevation of a solvent to the molal concentration of the unknown. If the compound is volatile, we can convert it to a gas and use its volume to determine the number of moles according to the gas law. Newer methods include mass spectrometry, which we will cover in Chapter 11. 

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