Heat of dilution from vapour pressure

To determine the differential heat of dilution of aqueous sucrose at mole fraction 0.1. 

Accurate values of the vapour pressure of aqueous sucrose solutions at 25 °C have been obtained by isopiestic (isotonic) equilibration with sodium chloride and potassium chloride solutions (compare problem 80), the values for the latter having been obtained by application of the Gibbs-Duhem relation to e.m.f. measmrements. Sudi % alues have been obtained by Scatchard, Hsuner, and Wood (J. Amer. Chem. Soc. 1988, 60, 3061) and by Robinson and Stokes (Trans. Faraday Soc. 1949, 45, 623). Direct vapour pressure measurements have been made by Berkeley (Phil. Trans. Roy. Soc. A 1919, 218, 344, 349) at 0 C and 30 C and by Dunning (private communication) at 60 °C and 90 C. 

It is convenient to express the relation of the vapour pressure j of the solution and the vapour pressure of the pure solvent at the same temperature by an osmotic coefficient. There are two such coefficients in use called the practical osmotic coefficient p and the rational osmotic coefficient g defined by (Robinson and Stokes, loc. cit.) 

If denotes the partial enthalpy of the solvent water in the solution and H the value of in the pure solvent, the differential heat of dilution J is given by 

We therefore plot q against IjT and from the slope obtain a value of d(pld ljT), which we use in (2). 

---