Steam and Immiscible Solvents Distillation

If the components of a solution decompose in some manner at its usual boiling point, then it must be distilled under less drastic conditions. One such method is a vacuum distillation discussed in Chapter 6. A second method that is much simpler, but is limited to those compounds that are immiscible with water, such as fats and oils, is a steam distillation. 

When immiscible liquids are heated, each exerts its own vapor pressure irrespective of the other. When the sum of the vapor pressures of these liquids becomes equal to the atmospheric pressure, they distill over together. Water is a preferred as one of the liquids, because it has a high vapor pressure, a low molecular weight, and is quite inexpensive. 

Suppose some carbon tetrachloride is mixed with the water, and the mixture is heated. We find that at about 68 C, the CCI4 has a vapor pressure of 550 torr and water has a vapor pressure of 210 torr. Together they have a vapor pressure of 760 torr, and the system will boil. The water will boil at 68 C rather than 100 C. It is the total pressure that is important. 

The preceding is in reality a statement of Dalton s Law of partial pressures, which states that the total pressure is equal to the sum of the partial pressures of the individual vapors. Therefore, the molar concentration is proportional to the number of moles present or  

The weight of each material is obtained by multiplying by the corresponding molecular weights. 

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