Which fraction of the drug will be in the lipid-soluble form in the extracellular fluids (pH = 7.4) if the pKa(negative log of acid) of lidocaine is 7.9?

24%-answer: b (katzung, pp 418, brunton, pp 374) all local anesthetics, but benzocaine, are weak bases and therefore they are mainly ionized when their pka is higher than the ph of the body fluids, like in the present case. the ratio between the non ionized and the ionized form can be found using the henderson-hasselbalch equation. when the difference between the pka and the ph is +0.5 the percentage of the ionized form is 76% and therefore the percentage of the non ionized, lipid soluble form will be 24%. a, c, d, e) (see explanation above)