Question

Acetic acid has the formula ch3cooh. At low ph this protonated, uncharged form is the most abundant form in solution. As the ph increases, more of the molecule becomes negatively charged acetate: ch3coo–. At which ph will more of the molecule cross the membrane?

Answer 1

Acetic acid is a weak acid. So, the undissociated acid molecules exist in equilibrium with the dissociated form. The equilibrium representing the weak acidic nature of acetic acid is,

$CH_{3}COOH(aq)+H_{2}O(l)CH_{3}COO^{-}(aq)+H_{3}O^{+}(aq)$

The interior of the cell membrane is hydrophobic. So, the membrane allows uncharged species (hydrophobic) to pass through it. Charged groups cannot easily pass through the membrane. We can say acetic acid can easily pass through the membrane when compared to the charged form acetate ion. At low pH, more of the molecule will cross the membrane as most of the acetic acid will be in undissociated and uncharged form at a lower pH.

$pK_{a}$of acetic acid is around 4.8. So at a pH value less than 4.8, we can say the more of the acetic acid molecules can cross the membrane.