Liquid molecular weight determination

The aluminum derivative of ethyl acetoacetate is a white crystalHne material, reported to melt at 76°, or 78 to 79°. It supercools readily from the melt to a straw-colored, very viscous liquid. Molecular weight determinations in carbon disulfide indicate that the compound is not associated in that solvent. The aluminmn derivative of ethyl acetoacetate is very soluble in benzene, ether, and carbon disulfide. It is less soluble in petroleum ether or cyclohexane and is insoluble in water. The compound boils at 190 to 200° at 11 mm. The reported dipole moment, in benzene, is 3.96 Debye. Surface tension and density values for the liquid above the melting point have been reported by Robinson and Peak. ... 

Molecular Weight Determination by Application of Raoult s Law. If a small amount (m in grams) of a nonvolatile, nonionized substance (solute, 2) is dissolved in m, grams of a volatile liquid (solvent, 1), it experiences a lowering of vapor pressure from the pure solvent value (P ) to the solution value (P) at the system temperature. This is a consequence of Raoult s law because the total vapor pressure of the dilute solution (x 1) is given by P = x P + x P = 1 -... 

One of the most elegant LS studies on liquid mixtures is that of Sicotte and Rinfret17 and it will be instructive to summarise their approach solely with regard to that aspect which is concerned with molecular weight determination. Liquid 1 will be considered as solvent and liquid 2 (of ostensibly unknown molecular weight M2) as solute. The Rayleigh ratios implied are the isotropic ones, which are obtained for liquid 1 as well as for solutions (subscript 12) via the measured Cabannes factors [Eqs. (44) and (45)]. 

The imstable seven-membered ring molecule disproportionates rapidly to form eight-membered SCjSj and a few minor components. The SejSj was characterized by chemical analysis, molecular weight determination, high-pressure liquid chromatography (HPLC), X-ray crystallography, and Raman spectroscopy and was shown to contain mainly the isomer 1,2,3-Se3S5 (see also 3.2-3.4). 

We recall from Chapter 10 that the percentages of the elements in a compound can be used to compute the simplest formula for the compound. When the substance is soluble in some suitable liquid, we can combine the empirical formula with a molecular-weight determination by freezing-point depression to get the true formula. 

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. 

Determining the molecular weights of asphaltenes is a problem because they have a low solubility in the liquids often used for determination. Also, adsorbed resins lead to discrepancies in molecular-weight determination, and precipitated asphaltenes should be reprecipitated several times prior to the determination (12). Thus, careful precipitation and careful choice of the determination method are both very important for obtaining meaningful results. 

One of the most important reactions of diborane is its interaction with amalgams of highly active metals, e.g. sodium and potassium, to give compounds such as KgBgHg, which are sometimes referred to as salts of diborane. These compounds are non-volatile, insoluble in liquid ammonia and common organic solvents and react with water. Consequently their formulae have been determined from magnetic measurements and not from molecular weight determinations. The addition of two electrons to the electronically unsaturated diborane,... 

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