Molecular mass, determination vapor-pressure method

An old method that is used occasionally is the vapor density method. In this method, a known volume of gas is weighed at a known temperature. After converting the volume of the gas to standard temperature and pressure, we can determine what fraction of a mole that volume represents. From that fraction, we can easily calculate the molecular mass of the substance. 

Another method of determining the molecular mass of a substance is to measure the freezing-point depression of a solvent that is brought about when a known quantity of test substance is added. This is known as a cryoscopic method. Another method, which is used occasionally, is vapor pressure osmometry, in which the molecular weight of a substance is determined through an examination of the change in vapor pressure of a solvent when a test substance is dissolved in it. 

Vapor Pressure Osmometry The VPO became a popular method for the determination of the number-average molecular weight of nonvolatile solutes of less than about 20,000 g/mol and that tend to diffuse across the membrane in MO experiments [91], This method operates on the principle that the vapor pressure of a solution is lower than that of a pure solvent (P°) at constant pressure and temperature. This vapor pressure lowering (AP) is proportional to the molar mass of the solute (polymer) for dilute solutions. As it is known, the vapor pressure of a solvent in dilute solutions obeys the Raoult s law, Pj = Pfxj, where Pj is the partial vapor pressure of the solvent whose mole fraction in the solution is Xj. In terms of the mole fraction of the solute, Pj = P° (l - X2) or AP/pO = -X2. 

Zeng et studied the self-assembly of dendrimers based on tetraacid systems. Eight scattering allowed them to determine the molecular weight of the dendrimers, with results that compared well with other methods, including size exclusion chromatography, mass spectrometry, cmd vapor pressure osmometry. 

Vapor-pressure lowering n. One of the col-ligative properties of a solution and the basis of a method for determination of the molecular mass of a solute. For a dilute solution, the solvent vapor pressure lowering is determined by (po—p)/po= 2> where po and p are the vapor pressures of the pure solvent and the solution, respectively, and X2 is the mole fraction of the solute. 

For samples with higher molecular weight (up to 3000 or more) with unusual composition or for polymers, ASTM Test Method D2503, Relative Molecular Mass (Molecular Weight) of Hydrocarbons by Thermoelectric Measurement of Vapor Pressure, is recommended. This method uses a vapor pressure osmometer to determine molecular weight. Low boiling samples may not be suitable if their vapor pressure interferes with the method. The method has only been standardized by ASTM for samples up to a molecular weight of 800. 

The most accnrate method for determining molecular weights is by high-resolution mass spectrometry (Section 9.17A). A variety of other methods based on freezing point depression, boiling point elevation, osmotic pressure, and vapor density can also be used to determine molecular weights. 

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