Ebullioscopic determinations molecular weight

The elevation of boiling point resulting from the addition to the solvent of a known weight of solute, enables the molality of the dissolved substance to be calculated, and hence the molecular weight of the solute if its weight concentration is known. This is the principle of the ebullioscopic method of determining molecular weights. 

Where M is the molecular weight of the solute, Dt is the elevation of boiling point in °C, c is the concentration of solute in grams for lOOOgm of solvent, and K is the Ebullioscopic Constant (molecular elevation of the boiling point) for the solvent. K is a fixed property (constant) for the particular solvent. This has been very useful for the determination of the molecular weights of organic substances in solution. 

The molecular weight and molecular weight distribution may be determined by conventional techniques. As the resins are of comparatively low molecular weight it is possible to measure this by ebullioscopic and by end-group analysis techniques. 

Poly-(methacrylic acid) was prepared in 1880 by Fittig and Engel-horn. Mjoen separated the polymer by precipitation and attempted to determine its molecular weight by cryoscopic and ebullioscopic methods. He decided that his product, which he regarded as a colloid, was an octamer but reached no conclusions as to its constitution other than that it was an octabasic acid of the formula C24H4o(COOH)8. 

In both cases the degree of dissociation increases with rising temperature. Ebullioscopic determination of the molecular weight shows that, in boiling benzene, 30 per cent of the substance is present as triphenylmethyl. 

The monochloride exhibits a slight tendency to molecular dissociation, but the effect is so small that both in the dissolved and in the gaseous condition the molecular weight agrees with the formula S2C12. As it does not combine with bromine, ebullioscopic determinations of... 

Polymeride of cycloselenohexane, (CgH Se). —The less soluble fraction mentioned in the foregoing preparation, when recrystallised from acetone yields the polymeride as a white, waxy, microcrystalline powder, M.pt. 36° to 37° C. Molecular -weight determinations by the ebullioscopic method indicate an aggregation of approximately twelve molecules of C6H12Se. The product yields a dibromide decomposing at 94° to 95° C. 

Titanium Complexes of Unsaturated Alcohols. Tetraallyl titanate can be prepared by reaction of TYZOR TPT with allyl alcohol, followed by removal of the by-product isopropyl alcohol. Ebullioscopic molecular weight determinations support its being the dimeric product, octaaUoxydititanium. A vinyloxy titanate derivative can be formed by reaction of TYZOR TPT with vinyl alcohol formed by enolization of acetaldehyde (11) ... 

When dissolved in organic liquids the molecular weight, as determined by ebullioscopic methods, appears to correspond to the single formula, CoBt2.5... 

The molecular weights of a number of arsinic acids have been determined by the ebullioscopic method, and generally the acids appear to have the simple, non-polymerised structure of the t3 ie... 

Association of solute molecules occurs when a substance like acetic acid, CH3COOH, is dissolved in a non-associated solvent such as benzene, and is evident from determinations of molecular weight by the cryoscopic or ebullioscopic methods. 

The ebullioscopic constant may be evaluated directly from equation 9 and thus provides an absolute method. However, the value for Tb is often determined by using a high purity solute of known molecular weight. 

Ebullioscopic molecular weight determinations show that vanadium tetramethoxide is trimeric. The tetraethoxide is dimeric, whereas the tetra-n-propoxide, n-butoxide and n-amyloxide show average degrees of association of 1.38, 1.31, and 1.27, respectively All secondary and tertiary alkoxides of vanadium are monomeric except the isopropoxide which shows shght association (1.17 complexity) in boihng benzene. 

The complex (Cl) was foimd to be cyclopentadienone-Fe(CO)3 while (CII) is postulated to be cydopentadienone-Fe(C0)2 54) or the dimer thereof (132) cryoscopic and ebullioscopic molecular weight determinations vary considerably in different solvents 54, 132, 133). Both (Cl) and (CII) give cyclopentadienone-triphosphine iron dicarbonyl when treated with tri-phenylphosphine the two are interconvertible by oxidation and carbonyl-ation reactions 133). 

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