Question

Chloroform is an excellent solvent for extracting caffeine from water. The distribution coefficient, KD, (Cchloroform/Cwater) for caffeine in chloroform-water at 25 oC is 10. What relative volumes of chloroform and water should be used for the extraction of 90 per cent of the caffeine from an aqueous solution in a single extraction

Answer 1

The relative volumes of chloroform and water that should be used is 9:10

Concentration of solution in chloroform = $90$ ( moles of chloroform )

Concentration of solution in water = $10$ ( moles of water )

Dissociation constant at $25^oC$; $K_D = 10$

$K_D =$ Concentration of solution in chloroform / Concentration of solution in water

Meaning;

$K_D = \frac{\frac{mole\ of\ chloroform}{volume\ of\ chloroform} }{\frac{mole\ of\ water}{volume\ of\ water} }$

Since $90$ mole is present in chloroform and $10$ mole is present in water, Total mole of Caffeine present is $100$

Now, we substitute our given values into the equation

$10 = \frac{\frac{90}{volume\ of\ chloroform} }{\frac{10}{volume\ of\ water} }\\\\10 *\frac{10}{volume\ of\ water} = \frac{90}{volume\ of\ chloroform} \\\\\frac{100}{volume\ of\ water} = \frac{90}{volume\ of\ chloroform}\\\\\frac{volume\ of\ chloroform}{volume\ of\ water} = \frac{90}{100}\\\\ \frac{volume\ of\ chloroform}{volume\ of\ water} = \frac{9}{10}\\\\ \frac{volume\ of\ chloroform}{volume\ of\ water} = 9:10$

Therefore, the relative volumes of chloroform and water that should be used is 9:10

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