Calculation and representation of saturated vapour pressures

If there is no opportunity to determine vapour pressures, it is possible to obtain the required data from figures published in the literature by the method of increments. This method, whether carried out graphically or by calculation, gives approximate results. 

Peter [47] investigated the relationship between vapour pressure and molecular structure on the basis of the van der Waals pressure equation  

By calculating the constant a additively for atomic and structural increments a good agreement with experimental results is obtained. In about half the cases investigated the difference between calculated and measured values of a amounted to less than 1 )er cent, and the variations in a due to temperature were small. B  

This rule hence states that the differences in boiling points at two pressures of two substances A and B bear a constant ratio. 

Taking water as a reference compound, as has been done by von Rechenbei [51] for a number of compounds (water being a substance with very accurately known vapour pressure data), we might arrive at the following calculation  

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