Acetic acid is an organic acid so it is a weak acid. Yet, it has a pH value of 3.9 which indicates it is a fairly strong acid. How is this possible?

Dear student,

Strong acids are those acids that will completely dissociate into their ions in water and they have a pH of between 0 to 1. 

Whereas weak acids are those acids that will not dissociate completely in water and they have a pH range of 2 to 7 depending on the concentration that we are using. 

1 M solution of acetic acid has a pH of 2.4 but it is a weak acid because it partially dissociates into its constituent ions when dissolved in water.

Regards