Methods based on transport properties

For molecular weights of the order of 10 or higher, none of the colligative properties can be measured accurately enough to provide reliable molecular weights. 

In such cases, alternative methods must be used. 

Information about the size of molecules is also provided by the results of experiments on the rates of movement of the molecules in solution. Properties which depend on rates of movement are referred to as transport properties. If the motion occurs in aqueous solution, we can also speak of hydrodynamic properties. The most important transport properties are diffusion, viscosity, and sedimentation. The theory of transport properties will be dealt with in Chapter 11, after the treatment of chemical kinetics. 

In the present section we will see, in a very general way, how measurements of transport properties lead to estimates of molecular sizes and shapes. 

The fundamental law of translational diffusion was formulated in 1855 by the German physiologist Adolf Eugen Pick (1829-1901) and is known as Pick s first law. 

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