Question

Please calculate the amount of a compound (A) that remains in 10 mL of an aqueous phase after three extractions with toluene (10 mL each) if the partition ratio (K_D) is

a) K_D = 9,   b) K_D = 3.   Show your calculations.

Answer 1

Apply the following relation to calculate the amount of substance left unextracted (x_n ) after n-th extraction,

x_n  = A ( K_D V / K_D V + v )ⁿ

where v represents ml. portion of the solvent used each time, supposing Vml of an aqueous solution contains A gms of an organic substance. K_D   is partition ratio.

Thus when K_D = 9 and assuming that amount of the substance in aqueous phase is = a gms in 10 ml of aqueous phase solution (ie V = 10ml. ) ; v ie. volume of organic ( toluene ) phase is given as 10ml.; n = 3 (given)

x_n   = a { ( 9 x10 ) / ( 9 x 10 ) + 10 }³

x_n  = 0.729 a

or in terms of % = 73% of the weight of the compound A originally present in aqueous phase.

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b ) Similarly,

when K_D  = 3

.....xn = a { 3 x 10 ) / (3 x 10 ) + 10 }³

.........= 0.42a

.....or,= 42 % of the original weight of the compound A present in 10 ml of aqueous phase.   

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