Two weak acids, HA (pK, = 3.0) and HB (pK, = 5.0) have equal partition coefficie

Question

Two weak acids, HA (pK, = 3.0) and HB (pK, = 5.0) have equal partition coefficients for extraction from water to n-octanol. An aqueous solution with equal concentrations of HA and HB is extracted with n-octanol. For equal volumes of water and n-octanol, which pH will provide the best separation of HA and HB? (A) 2.0 (B) 3.0 (C) 4.0 (D) 5.0 4 5 . An example of a nonaqueous solvent for normal-phase liquid chromatography is (A) methanol. 46. (B) hexane. (D) sodium dodecylsulfate. (C) aqua regia. Which compound is expected to have the greatest retention time in a reversed-phase HPLC column? 47. (A) sodium acetate, CH,COONa (B) phenol, CH,OH (C) butanoic acid, CH,CH CH COOH (D) n-dodecane. CH(CH CH

Explanation / Answer

we must choose a pH:

pH 2 and 3 --> HA is protonated, a well as HB

if pH = 4

then

A, has been deprotonated mostly, A-

HB remains protonated, mostly, so

this is the optimal pH

betwen pKa data

Q46

nonaqueous solvvent --> hexane, since it is nonpolar, will not bee in aqueous solutoin

all other are soluble in water

Q47

greatest retention time --> the most "nonpolar", from the list, choose n-dodecane

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