Suppose you have a solution of a new drug molecule that is a weak acid. The mole

Question

Suppose you have a solution of a new drug molecule that is a weak acid. The molecule has only one ionizable group in its structure. When this group deprotonates, it becomes an anion and upon protonation it becomes neutral. In this solution which is buffered to pH 7.4 to match the chemistry of the blood, you intend to perform some physiochemical analysis using NMR. Upon analysis you discover that exactly 50.00% of the total drug molecules are deprotonated and the remaining 50.00% are protonated.

What is the pKa of this drug? (give your answer to 1 decimal place like this: 1.2)

Explanation / Answer

pH = pKa + log([A]- / [HA] )

pH given is 7.4;

[A]- = 50%

[HA] = 50%

Therefore log 1 = 0

Then pH becomes pKa

pKa = 7.4

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