Only about 36 g of CH 3CO 2Na are soluble in 100 g of water at 0☌, however, so approximately 114 g (150 g − 36 g) of CH 3CO 2Na crystallizes out on cooling. According to the temperature curves in Figure 13.9 "Solubilities of Several Inorganic and Organic Solids in Water as a Function of Temperature", both compounds dissolve in water at 80☌, and all 50 g of KBr remains in solution at 0☌.
For example, if we have a mixture of 150 g of sodium acetate (CH 3CO 2Na) and 50 g of KBr, we can separate the two compounds by dissolving the mixture in 100 g of water at 80☌ and then cooling the solution slowly to 0☌.
Chemists are often able to use this information to separate the components of a mixture by fractional crystallization The separation of compounds based on their relative solubilities in a given solvent., the separation of compounds on the basis of their solubilities in a given solvent. The variation of solubility with temperature has been measured for a wide range of compounds, and the results are published in many standard reference books. Because the solubility of a compound is ultimately determined by relatively small differences between large numbers, there is generally no good way to predict how the solubility will vary with temperature. In fact, the magnitudes of the changes in both enthalpy and entropy for dissolution are temperature dependent. The dissolution of ammonium nitrate in water is endothermic (Δ H soln = +25.7 kJ/mol), whereas the dissolution of calcium chloride is exothermic (Δ H soln = −68.2 kJ/mol), yet Figure 13.9 "Solubilities of Several Inorganic and Organic Solids in Water as a Function of Temperature" shows that the solubility of both compounds increases sharply with increasing temperature. Notice in particular the curves for NH 4NO 3 and CaCl 2. Solubility may increase or decrease with temperature the magnitude of this temperature dependence varies widely among compounds.
Figure 13.9 Solubilities of Several Inorganic and Organic Solids in Water as a Function of Temperature
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