During an extraction, a solute (S) will partition itself between the two solvent

Question

During an extraction, a solute (S) will partition itself between the two solvent phases with a Partition Coefficient (K)-defined as: K = C_2/C_1, where C_1 and C_2 are the concentrations at equilibrium, in grams per liter of the solute (S) in solvent 1 and in solvent 2. respectively. Solute (S) is found to have a partition coefficient of 12. Suppose 5 g of (S) is dissolved in 150 mL of solvent 1 and the solution is extracted with 50 mL of solvent 2. How much of S remains in solvent 1? Steam distillation is a valuable technique for the isolation of many natural products from plant material. Explain why this technique would fail if you tried to steam distill caffeine from lea leaves. Using the concept of partial pressures, explain why the boiling point of a steam distillation is always less than 100degreeC.

Explanation / Answer

3) 5 g of S is dissolved in 150 mL of solvent 1

x goes to solvent 2

5-x remains in solvent 1

5-x/150/x/50 = 12

5-x/3/x = 12

5-x/3x = 12

5-x = 36x

5 = 37x

x = 5/37

x = 0.135

0.135 g goes to 50 mL of solvent 2

5-0.135 = 4.864 g remains in solvent1

4) Steam distillation principle is as follows

When a mixture of two practically immiscible liquids is heated, each constituent independently exerts its own vapor pressure as a function of temperature as if the other constituent were not present. Consequently, the vapor pressure of the whole system increases. Boiling begins when the sum of the vapour pressures of the two immiscible liquids just exceeds the atmospheric pressure.

Caffeine on the other hand is soluble in hot water to a significant extent. Caffeine is moderately soluble in water at room temperature (2 g/100 mL), but very soluble in boiling water (66 g/100 mL). SO steam distillation of this is not possible.

5) As mentioned earlier the concep of steam ditillation hinges on the fact that the two material should be immiscible.

When a mixture of two practically immiscible liquids is heated , each constituent independently exerts its own vapor pressure as a function of temperature as if the other constituent were not present. Consequently, the vapor pressure of the whole system increases. Boiling begins when the sum of the vapour pressures of the two immiscible liquids just exceeds the atmospheric pressure. In this way, many organic compounds insoluble in water can be purified at a temperature well below the point at which decomposition occurs.

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