For a drug that is a weak base with a pKa of 8.4, the predominant species at pH 7.4 will be: 

Question

For a drug that is a weak base with a pKa of 8.4, the predominant species at pH 7.4 will be:

BlackTom AI · Accepted Answer

First, let's identify what the question is asking: at pH 7.4, for a drug that is a weak base with a pKa of its conjugate acid (8.4), which form is predominant?
Option 1: Positively charged. This is the correct choice. For a weak base, the protonated form (BH+) is positively charged. Since pH 7.4 is one unit below the conjugate acid pKa of 8.4, Henderson–Hasselbalch for bases tells us the ratio of unprotonated base (B) to protonated form (BH+) is [B]/[BH+] = 10^(pH - pKa) = 10^(7.4 - 8.4) = 10^(-1) = 0.1. That means most molecules are in the BH+ form, i.e., positively charged.
Option 2: Uncharged. This would require a larger fraction of B relative to BH+. The calculated ratio (0.1) means only about one part B for ten parts BH+, so the uncharged form is not predominant at pH 7.4.
Option 3: Negatively charged. Protonation of a base yields BH+. Deprotonation to form a negatively charged species would require the molecule to lose a proton below neutral pH, which is not supported by the base’s behavior here. The equilibrium at pH 7.4 does not favor a negative charge for a weak base with pKa 8.4.
Option 4: Zwitterionic. A zwitterion would require simultaneous positive and negative charges on different sites in the same molecule. The Henderson–Hasselbalch data here do not indicate formation of a structure carrying both charges; the dominant form is simply BH+ (positively charged) with the neutral base being minority.
In summary, because the solution pH is lower than the base’s conjugate acid pKa, the equilibrium favors protonation to BH+, yielding a positively charged predominant species. The other options misinterpret protonation balance, charge state, or the possibility of internal charge separation.

PCOL2021 2025 Practice MCQ Section A - 2025

For a drug that is a weak base with a pKa of 8.4, the predominant species at pH 7.4 will be:

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