The ideal solubility of solids in liquids

Let it be supposed that this equation continues to hold, at least approximately, up to a mole fraction of unity. This corresponds to point A in Fig. 37, where the pure liquid solute is in equilibrium with 

Equation (8 63), which was first put forward by Schroeder in 1893, may be used to estimate the ideal solubility of solids in liquids, from a knowledge of their melting-points and latent heats. It will be noted that the equation is entirely analogous to (8 48) which gives the depression of the freezing-point of the solvent a more accurate form of (8 63) could have been obtained by allowing for the temperature dependence of , as in 8 9. 

As an example, the enthalpy of melting of naphthi lene is 18 580 J mol and its melting-point is 80.05 C. Its ideal solubility at 20 may therefore be calculated from the above equation to be a =0.273 and is the same in every solvent. Some of the experimental data for solvents of low polarity are given in the table.f 

As deductions from equation (8 63) we have (a) the solubility of a solid may be expected to increase with rise of temperature (6) the solubility of a solid may be expected to be the greater the lower is its melting-point and the smaller is its enthalpy of melting. These results, although they are based on the supposition of ideal solutions, are in fairly general agreement with experience. 

It was shown by Zawidski that mixtures of benzene and ethylene chloride obey Raoult s law quite accurately. At 50.0 0 their vapour pressures as pure liquids are 268.0 and 236.2 mmHg respectively. At this temperature calculate the total pressure emd the composition of. the vapour which is in equilibrium with the liquid containing mole fractions of 0.25, 0.50 and 0.75 of benzene. 

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For the ideal solubilities of gases in liquids, a similar approach to that taken in Section 3.1 for the ideal solubilities of solids in liquids can be used and thus, Equation (3.69), analogous to Equation (3.8), is obtained ... 

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