Need help with some homework. Penicillin V: pKa= 2.79, C 16 H 18 N 2 O 5 S 350.3

Question

Need help with some homework.

Penicillin V: pKa= 2.79, C16H18N2O5S 350.39 g/mol

5.0 g of Penicillin V was shaken with a mixture of 10 mL of diethyl ether and 10 mL of water. The concentration of the analyte was found to be 1.23 mol/L in diethyl ether and 0.0100 mol/L in water.

a) Calculate the distribution coefficient (D) for Penicillin V.

b) If 10g of Penicillin V had been shaken with 50mL of each solvent, what value would you expect for D if the layers were again analyzed?

(my professor said the answers for a & b were the same)

c) If 2.0 g of penicillin V was dissolved in 100 mL of a 1:1 diethyl ether:water mixture, what mass (in grams) of the drug would be extracted into the diethyl ether layer?

Explanation / Answer

Distribution coefficient (D) = COncentration in organic solvent / Concentration in water

D = (1.23 mol/L)(diethyl ether) / ( 0.01 mol/L)(water)

D = 123

If 10 g of Penicillin V is used with 50 mL of water

mol of Penicillin = 10/350.39 = 0.0285

Concentration in diethyl ether = 28.5 /1.426* 50 = 0.4 mol/L

Concentration in water = 28.5 / (142.6*50) = 0.004 mol/L

D = 0.4 / 0.004 = 100

Mass of drug extracted = 2 / 1.426 = 1.402 g

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