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Why we do three organic extractions instead of one

When an aqueous solution of an organic compound is shaken with an immiscible organic solvent, such as diethyl ether, the solute distributes itself between the two phases. When the two phases separate into two distinct layers, an equilibrium will have been established such that the ratio of the concentrations of the solute in each solvent defines a constant, K, called the distribution coefficient.

K= (concentration of solute in solvent A, e.g. diethyl ether (g L-1)/concentration of solute in solvent B, eg. Water (g L-1)

The distribution coefficient for compound X in the diethyl ether/water system is 3.0. If you were given a solution containing 8.0g of X in 500ml of water, and wanted to extract compound X into diethyl ether, show that it would be more effective to extract X using three 50ml aliquots of diethyl ether rather than a single 150mL aliquot. (Determine how much of X would remain in the aqueous solution in each case)

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Solution Preview

We can treat this like an equilibrium problem. The initial concentration of the compound is 16 g/L in the 500 mL of water. When we extract, we will lose some amount x to the diethyl ether. Using an ICE table
H2O Et2O
Initial ...

Solution Summary

In this solution the concept of extraction is explored. Specifically, why is it more efficient to extract with three small volumes rather than one large volume? A mathematical description along with a simplified explanation is contained here in an effort to help this important concept make as much sense as possible!