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SolutionsExtraction

1. One extraction or two

A compound has a partition coefficient K = 4 between ether and water. You have 1.0 g of the compound dissolved in 100 mL of water. How much do you extract with a single extraction with 100 mL of ether? And with two successive extractions with 50 mL?

If x g remains in the water: K = [(1 − x)/Vorg] / [x/Vaq].

  1. With 100 mL: 4 = (1 − x)/x, so x = 0.20 g; 0.80 g is extracted.
  2. With 50 mL: 4 = [(1 − x)/50]/(x/100), so x = 1/3 in each extraction. After two, 1.0 × (1/3)2 = 0.11 g remains in the water; 0.89 g is extracted.

With the same total volume of solvent, several small extractions are more efficient than a single large one.

2. Top or bottom?

In a separatory funnel with water, where is the organic phase if the solvent is: a) diethyl ether (density 0.71 g/mL); b) dichloromethane, CH2Cl2 (density 1.33 g/mL)?

  1. On top: ether is less dense than water.
  2. At the bottom: dichloromethane is denser than water.

3. Separating three compounds

An ether solution contains benzoic acid, aniline and naphthalene. Suggest how to separate them by acid-base extraction.

  1. Wash with aqueous HCl: aniline passes into the water as anilinium chloride; making that phase basic with NaOH releases the aniline.
  2. Wash the ether phase with aqueous NaOH (or NaHCO3): benzoic acid passes into the water as sodium benzoate; acidifying with HCl precipitates it.
  3. Naphthalene stays in the ether and is recovered by evaporating the solvent.

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