Freezing point, molecular lowering

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 physical properties normally needed in design and production are vapor pressure, heat of vaporization, density, surface tension, heat capacity and thermal conductivity. For chemical reactions, enthalpy of formation and Gibb s free energy of formation are helpful. Also, the boiling point, freezing point, molecular weight, critical properties, lower explosion limit in air and solubility in water are tabulated for each compound. This latter property data are helpful in safety and environmental engineering. 

When 0.750 gm. of a substance was dissolved in 22 gm. water the freezing point was lowered 0.105°. Calculate the molecular weight of the compound. 

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. 

Table 11.2 Molecular Lowering of the Melting or Freezing Point. 2 DRYING AND HUMIDIFICATION...

M depends not on the molecular sizes of the particles but on the number of particles. Measuring colligative properties such as boiling point elevation, freezing point depression, and vapor pressure lowering can determine the number of particles in a sample. 

The presence of a solute affects the physical properties of the solvent. For instance, when salt is spread on icy sidewalks, a mixture is created with a lower freezing point than that of pure water and the ice melts. In this part of the chapter we explore the molecular nature of these effects and see how to treat them quantitatively. 

The addition of 0.24 g of sulfur to 100. g of the solvent carbon tetrachloride lowers the solvent s freezing point by 0.28°C. What is the molar mass and molecular formula of sulfur ... 

From these results it is possible to make another estimate of a property of the solution system. It is known that the freezing point of a solvent is lowered by approximately 1.86°C for every mole of the solute present. From the estimates of the temperature of the solvent and the solution modeled above, the decrease in the temperature can be estimated. From this value, the number of cells comprising a mole of solute may be reckoned. Thus, a value may be stated for an imaginary molecular weight of the cells used in the study. 

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 colligative properties of solutions are those properties that depend upon the number of dissolved molecules or ions, irrespective of their kind. They are the lowering of the vapor pressure, the depression of the freezing point, the elevation of the boiling point, and the osmotic pressure. These properties may be used in determining molecular weights of dissolved substances. 

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