Molecular mass, determination freezing-point method

The Determination of Molecular Mass by the Freezing-Point Method... 

The freezing-point method is a very useful way of determining the molecular masses of substances in solution. Camphor, with its very large constant, is of particular value for the study of organic substances. 

For a molecular compound, the full molecular formula can be established from the empirical formula and the molecular mass (RMM). Various physical properties, including the vapour density of a gas, and so-called colligative properties (such as freezing point depression) in solution, can be used to determine the RMM. However the most important technique in modem research is mass spectrometry (MS) where molecular ions are accelerated in an electric field, and then pass through a magnetic field where their paths are bent to an extent that depends on the mass/charge ratio. The traditional MS method requires a volatile sample, ionized by electron bombardment, but methods are now available that overcome the limitations of that method. Direct desorption from solids by a laser beam or by fast atom bombardment (FAB) allow measurement of involatile compounds. Solutions may also be sprayed directly into the spectrometer inlet and the spectrum measured after the solvent has evaporated. 

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. 

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. 

Calculation of the Molecular Formula How do we know the correct molecular formula We can choose the right multiple of the empirical formula if we know the molecular weight. Molecular weights can be determined by methods that relate the freezing-point depression or boiling-point elevation of a solvent to the molal concentration of the unknown. If the compound is volatile, we can convert it to a gas and use its volume to determine the number of moles according to the gas law. Newer methods include mass spectrometry, which we will cover in Chapter 11. 

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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