Freezing point depression analysis

Colligative properties are dependent on the number of particles present and are thus related to M . M values are independent of molecular size and are highly sensitive to small molecules present in the mixture. Values of are determined by Raoult s techniques, which are dependent on colligative properties such as ebulliometry (boiling point elevation), cryometry (freezing point depression), osmometry, and end-group analysis. 

See also Analysis (Chemical) and Freezing-Point Depression. 

The polyformal (I) was characterized by wet chemical and infrared analyses, and by cryoscopic molecular weight determinations. For the latter, the preferred method was the freezing point depression of an ethylene bromide solution. The other polyacetals were characterized primarily by infrared analysis and molecular weight determinations. 

Elemental analysis yields Cio Hi4.9oO (or possibly CjoHisO), or OiHmO. The freezing point depression calculation yields a MW of 313.8, which agrees with CjiHsoOf (actual MW = 314.45). 

MNDO calculations, spectroscopic investigations, and X-ray analysis. It was also shown by freezing point depression experiments that the lithiated SAMP hydrazones exist in a monomeric form. Electrophilic attack by the electrophile on this system proceeds from the sterically more accessible face with high diastereoselectivity. 

In principle, one would also expect the homocyclic Sg to be present in hot sulfur melts but this species may be too unstable to survive the quenching and extraction procedure since never has ever a peak at the expected retention time been observed. However, according to Rau et al. [94] saturated sulfur vapor contains only 0.7 mol% Sg but 20% Sg and 12% Sy at 200 °C. At higher temperatures the melt will also contain traces of S4, S3 and S2 [61] see also the next section. The sum of the concentrations of the non-Sg rings at 116 °C is 4.87%, practically identical to the concentration of r-sulfur as derived from the freezing point depression (see above), thus confirming the reliability of the HPLC analysis. 

The elemental analysis of an organic solid extracted from gum arabic (a gummy substance used in adhesive, inks, and pharmaceuticals) showed that it contained 40.0 percent C, 6.7 percent H, and 53.3 percent O. A solution of 0.650 g of the solid in 27.8 g of the solvent diphenyl gave a freezing-point depression of 1.56°C. Calculate the molar mass and molecular formula of the solid. K for diphenyl is 8.00°C/ot.)... 

The starting point for the analysis of the freezing-point depression phenomenon is the observation that when a solid freezes out from a mixture, it generally is pure and it is the component that is concentrated in the solution, that is, the pure solvent. [Indeed, this is the basis for zone refining, in which melting and resolidification (or recrystallization) are used to purify metals.] The equilibrium condition when the first crystal of pure solvent forms is... 

The starting point for the analysis of the freezing point depression is ... 

The determination of the molecular weight has also often led to difficulties. Freezing-point depression, diffusion and sedimentation in the ultracentrifuge have sometimes led to discrepancies owing to association or solvents of crystallization. The partial substitution method of Battersby and Craig , the thermoelectric method of Simon and Tomlinson , mass-spectrometric measurements and x-ray analysis have been used recently to overcome such difficulties. 

At one time, the molecular weight of a compound was determined by its vapor density or its freezing-point depression, and molecular formulas were determined by elemental analysis, a technique that determined the relative proportions of the elements present in the compound. These were long and tedious techniques that required relatively large amounts of a very pure sample. Today, molecular weights and molecular formulas can be rapidly determined by mass spectrometry from a very small amount of a sample. 

Osmotic pressure, equilibrium centrifugation, end group analysis, freezing point depression (cryoscopy), boiling point elevation (ebulliometry) and vapor pressure lowering measure the colligative... 

Infrared spectroscopy gives information about the functional groups in a molecule, but it tells little about the size of the molecule or what heteroatoms are present. To determine a structure, we need a molecular weight and a molecular formula. Molecular formulas were once obtained by careful analysis of the elemental composition, and a molecular weight was determined by freezing-point depression or some other technique. These are long and tedious processes, and they require a large amount of pure material. Many important compounds are available only in small quantities, and they may be impure. 

It was argued by some, early on in the story, that pol5water was simply impure water since boiling point elevation and freezing point depression are colligative properties characteristic of solutions. However, the flurry of papers emphasized the precautions that had been taken to avoid contamination, so for several years the existence of polywater was taken for granted as a real phenomenon. However, eventually the case for polywater began to crumble. Because it could only be formed in quartz capillaries of very small internal diameter, very little was available for analysis. When eventually small samples could be subjected to trace chemical analysis, polywater was shown to be contaminated with a variety of substances from silica to phospholipids. Moreover, electron microscopy revealed that polywater also contained finely divided colloidal particulates in suspension. 

Other important parameters in particle analysis are surface area, pore size, and volume. The basic method for measuring surface area involves determining the quantity of an inert gas, usually nitrogen, required to form a layer one molecule thick on the surface of a sample at cryogenic temperature. Many techniques are used for pore size measurement impregnation with molten metal, particle beam transmission, water absorption, freezing point depression, microscopy, mercury intrusion, and gas condensation and evaporation. The last three techniques are most often utilized. 

An unknown compound contains only carbon, hydrogen, and oxygen. Combustion analysis of the compound gives mass percents of 31.57% C and 5.30% H. The molar mass is determined by measuring the freezing-point depression of an aqueous solution. A freezing point of —5.20°C is recorded for a solution made by dissolving 10.56 g of the compound in 25.0 g water. Determine the empirical formula, molar mass, and molecular formula of the compound. Assume that the compound is a nonelectrolyte. 

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