Question

You are given a substance that is equally soluble in water and hexane (two immiscible solvents). a.) If you start with 10 g of the substance dissolved in 100mL of water and are given 100mL of hexane to extract with how much will you extract into hexane if you do a single extraction with all 100mL of the hexane? b.) If you start with 10g of the substance dissolved in 100mL water and are given 100mL of hexane to extract with how much will you extract into hexane if you do two smaller extractions using 50 mL of hexane for each extraction?

Answer 1

The tendency of a substance to dissolve in a system of two immiscible solvents is called the partition coefficient. If the substance given is equally soluble in both water and hexane, irespective of the quantity of hexane used, if all 100mL of hexane is used in one-go, the 10g of substance will distribute itself equally between the two solvents due to its partition coefficient in the given solvent system being the same. However, if 50mL batches of hexane is carried out, the extract will even then have the substance distributing itself equally between the two solvents every time hexane is added.

Thus, if all 100mL is used in a single extraction, only 5g of the substance can be extracted. However, if two 50mL extractions are done, the first extraction will contain 5g and the next extraction will contain the remaining 5g of the substance distributed equally between hexane and water, leaving only 2.5g in the second extraction, giving a total of 7.5g of substance in the second case. NOTE: All calculations are done considering the substance is freely soluble in both solvents and that 100mL of solvent is a large excess for 10g of the substance.

So, for case a) 5g of substance will be extracted and in case b) 7.5g will be extracted into hexane.