If a weak acid is 25% deprotonated at ph 4, what is the pka? if a weak acid is 25% deprotonated at ph 4, what is the pka?

pKa = 4.5

Explanation:

The weak acid can be represented by the general formula, HA and the dissociation equilibrium given as:

where HA = protonated form

A- = deprotonated form

The Henderson-Hasselbalch equation relates the pH of a solution to the ratio of the concentrations of HA and A- as;

It is given that:

% deprotonated i.e. A- = 25%

Therefore, %protonated i.e. HA = 100 -25 = 75%

pH = 4

Based on equation (1)

pKa = 4.477 i.e. around 4.5