Question

Use the Henderson-Hasselbalch equation and your knowledge of ionization to help you answer this question. Aspirin is a weak acid with a pKa of 3.5 that is absorbed more effectively in the stomach than the small intestine. The pH of your stomach is around 1.5 and the pH of your small intestine is approximately 6.0. Is aspirin absorbed more readily when it is protonated or deprotonated? What is the approximate ratio of conjugate base to acid when it is absorbed more readily?

Answer 1

Answer:

The protonated form is predominant when aspirin is absorbed more readily. The ratio of conjugate base to acid is 1 to 100.

Explanation:

Aspirin is more readily absorbed when it is protonated, that is when pH is lower than pKa (more H⁺ available in the medium). We can confirm this using Henderson-Hasselbalch equation for pH = 1.5:

$pH = pKa +log(\frac{conjugate base}{weak acid} )\\1.5=3.5 +log(\frac{conjugate base}{weak acid} )\\-2.0=log(\frac{conjugate base}{weak acid} )\\10^{-2.0} =\frac{conjugate base}{weak acid}\\\frac{conjugate base}{weak acid}=0.01=\frac{1}{100}$

When aspirin is absorbed more readily the ratio of conjugate base to acid is 1 to 100, being the acid the predominant form.