when performing extractions it is often better to do multiple extractions rather
ID: 739928 • Letter: W
Question
when performing extractions it is often better to do multiple extractions rather than one large volume extraction....suppose you have 100mg of a sample dissolved in 0.6ml of water. the partition coefficient or kp for this compound in water and diethyl ether is 4.0
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a. calculate how much total compound you would obtain with 3 x 0.2ml extractions with ether
b. calculate how much you would obtain with 1 x 0.6ml extraction
c. which method is more efficient?
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Explanation / Answer
Partition coefficient of 4 means concentration in the ether layer will be 4x the concentration in the water layer. So first extraction: (Conc in ether)/(Conc in water) = 4 (x mg in ether / 0.2 mL ether)(50-x mg in water / 0.6 mL water) = 4 x = 4(0.2 mL)(50-x mg)/0.6 mL = 66.7 - 1.33 x 2.33 x = 66.7 x = 28.6 mg Since each extraction uses the same ether volumes, calculate the fraction of sample left in the water after 1 extraction, and cube this to find the fraction left after three extractions. Amt left in water after 1 extraction = 50 - 28.6 = 22.4 mg Fraction left in water after 1 extraction = 22.4 mg / 50 mg = 0.428 Fraction left in water after 3 extractions = (0.428)^3 = 0.0785 Amt left in water after 3 extractions = (50 mg)(0.0785) = 3.9 mg Amt extracted after 3 extractions = 50 - 3.9 = 46.1 mg
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