Measurement of Fugacity

It is obvious from the preceding equations that to measure fugacity, we need to measure the molar volume as a function of P. In other words we have to measure gas densities. Also we should measure these densities down to very low pressures— pressures sufficiently low that we can say / = P to whatever degree of accuracy is required. The greater the accuracy the lower the pressure. But at these low pressures, the volume of a given mass of gas becomes very large and very difficult to measure 

Thus a is the difference between the molar volume of the gas and the volume it would have if it behaved ideally. Then 

Equation (11.9) is also useful in providing a one-equation definition of fugacity, rather than the two-equation one we gave above. Substituting (RT/P — 1 ) for a and rearranging, we have 

This is the equation given by Tunell in his classic paper on fugacity (1931), in which he points out some difficulties with the definition as we have presented it, and shows that equation (11.10) is preferable as a definition of fugacity from a mathematical point of view. Our presentation, which follov s the original one by Lewis and Randell (1923), seems preferable from a pedagogical point of view. 

The introduction of the a function should not be allowed to obscure the fact that fugacity is obtained by evaluating the function / V dP. The a function only simplifies this evaluation. 

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