260 ILLINOIS STATE ACADEMY OF SCIENCE 



calculate the freezing point cnrve or the solubility of any 

 substance in the table with any other substance in the 

 table providing the latent heat of fusion (ideal slope 

 of the log N vs. 1/T curves) is known and the assumption 

 is warranted that there are no complicating molecular 

 effects. In making the calculation all that is necessary is 

 to find the factor by which the ideal slope must be in- 

 creased in order to make it equal to the slope which 

 would be determined experimentally. This factor is 

 found by placing a straight edge across the chart in such 

 a position that it joins the components of the desired 

 system. The point at which this cuts the line of factors 

 will then be the ratio sought. Evidently, the nearer two 

 substances are to one another in the table, the more 

 nearly will their reciprocal solution approach to that of 

 the ideal mixture. A comparison of columns 4 and 5, 

 Table 1, will indicate the degree of precision to be ex- 

 pected. 



In determining molecular weights by the melting point 

 or boiling point methods, one should choose a solvent 

 which has approximately the same position in .the chart 

 as the solute to be used. If this rule is not followed 

 it will be observed that the '* molecular association" of 

 the solute will appear to increase in direct proportion to 

 the magnitude of the factor relating the solute and sol- 

 vent in the above chart. 



In this connection, it is commonly assumed that acetic 

 acid in benzene solution (benzene = solute) is associated 

 into double molecules as determined from the Van't Hoff 

 freezing point laws. In making that assumption, the 

 further assumption has been made that the heat of the 

 solution process is equal to the latent heat of fusion. In 

 many ways it is more tenable to assume that in those 

 systems which give fairly straight logarithmic solubility 

 curves, at least, the variation from the normal is in the 

 heat effect rather than in the moleciilar weight of the 

 dissolved substance. This view is further supported 

 by the fact that when molecular complexes are formed 

 very highly curved solubility curves are obtained. 



This chart should also aid in the choice of solvent to 

 be used in recrystallization and in the choice of extract- 



