Comparative Freezing Point Depression

Freezing point curve for Sulfolane — water mixtures 

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Compare the freezing point depression values you calculated to the... 

In contrast to the simple olefins, aryl-substituted olefins dissolve in sulfuric acid to give comparatively stable carbonium ions, as is shown by the -factors, the spectra, and the recovery of the olefin on dilution.262 In some cases it is neccessary to extrapolate the freezing point depression to zero time owing to a slow sulfonation. Because of the similarity in the spectra it is believed that these carbonium ions have the classical structures shown below.263... 

Colligative properties reflect the chemical potential of the solvent in solution. Alternatively, a colligative property is a measure of the depression of the activity of the solvent in solution, compared to the pure state. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and membrane osmometry. The latter property is considered here, since it is the most important of the group as far as synthetic polymers are concerned. 

An alternative approach is based on the theoretical foundation described earlier for the colligative properties. If the solution is isotonic with blood, its osmotic pressure, vapor pressure, boiling-point elevation, and freezing-point depression should also be identical to those of blood. Thus, to measure isotonicity, one has to measure the osmotic pressure of the solution and compare it with the known value for blood. However, the accurate measurement of osmotic pressure is difficult and cumbersome. If a solution is separated from blood by a true semipermeable membrane, the resulting pressure due to solvent flow (the head) is accurately measurable, but the solvent flow dilutes the solution, thus not allowing one to know the concentration of the dissolved solute. An alternative is to apply pressure to the solution side of the membrane to prevent osmotic solvent flow. In 1877, Pfeffer used this method to measure osmotic pressure of sugar solutions. With the advances in the technology, sensitive pressure transducers, and synthetic polymer membranes, this method can be improved. However, results of the search for a true semipermeable membrane are still... 

One mole of solute dissolved in 1000 g of solvent (to produce a 1 molal solution) depresses the freezing point by the molal freezing point depression constant (X/). The degree to which the freezing point of any solution is depressed (compared to that of the pure solvent) is directly proportional to the molality of the solute present. 

The expressions for boiling-point elevation and freezing-point depression apply accurately to dilute solutions only. A saturated aqueous solution of Nal (sodium iodide) in water has a boiling point of 144°C. The mole fraction of Nal in the solution is 0.390. Compute the molality of this solution. Compare the boiling-point elevation predicted by the expression in this chapter with the elevation actually observed. 

In clinical laboratories, the vapor pressure osmolality technique has been reported to be less precise than the freezing point depression method. For serum samples, the coefficients of variation obtained for the vapor pressure osmometer are about twice those obtained for the freezing point depression osmometer. The lesser degree of precision is related to the lower slope of dew point decrease compared with freezing point decrease (i.e., 0.303 °C versus 1.86 °C per osmol/kg H2O). 

To evaluate the van t Hoff factor, i, we first calculate effective frotn the observed freezing point depression and for water we then compare effectiTe stated... 

GB is very hygroscopic and shows remarkable freezing-point depression at high concentrations compared with other compatible solutes. Tg value for the freeze-concentrated GB system was very low and the amount of unfrozen water is large. These characteristics of GB, ascribed to those of zwitterions with trimethylammonium ions, might contribute to the protective fimction against drought, salt, or low-temperature stress. 

If it is assumed that an absorbed layer of water exists before capillary condensation takes place and that this layer consists of ordered or oriented water molecules, then the contact angle in Kelvin s equation should be very close to zero. With zero contact angle, vapor pressures in the capillaries are calculated from Kelvin s equation for capillaries from 10 %. to 200 X, Table I. The vapor pressure of water below 0 C (15) is compared with the vapor pressures in the capillaries to obtain the freezing points. Figure 1 shows the relation between the freezing point depression and the capillary radius. 

The freezing point depression constant for cyclohexane is 20.3, which is very high when compared to the 1.86 for water. You need 20-30 mL of pure material in order to make several measurements, and all you have available is some technical-grade material that was prepared by catalytic hydrogenation. You are to do an extractive distillation to remove the benzene impurity. 

Freezing-point depression. The lowering of the freezing point of a solution (compared to that of the pure solvent) due to the presence of a solute. 

An interesting attempt to distinguish between the occurrence of open or cyclic dimers has been made by Josien et al. (5), who have carried out infrared and cryoscopic measurements at the same temperature on the same dilute solutions of a number of alcohols in Freon 112 (1,1,2,2-tetrachloro-1,2-difluoroethane). From the cryoscopic data a mean degree of association, x, is defined as x = x/xp, where x is the mole fraction of the dissolved alcohol and xp is the mole fraction of particles (monomer and multimer) indicated by the freezing point depression. The quantity x is compared with 0, where 1/0 is the fraction of free OH groups from the infrared spectra,... 

Two grams of benzoic acid dissolved in 25 g of benzene, Xy = 4.90 Kkg/mol, produce a freezing-point depression of 1.62 K. Calculate the molar mass. Compare this with the molar mass obtained from the formula for benzoic acid, CgHjCOOH. 

What is the direction of the influence of nonideality (for example, positive deviations from Raoult s law) on (a) freezing-point depression, (b) boiling-point elevation, and (c) osmotic pressure compared to the ideal solution case ... 

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