Warum ist die Chloressigsäure eine schwache Säure?

Chloroacetic acid has pKa=2.86, which is clearly not a strong acid (which would have to be completely dissociated, pKa <0), but also not really weak anymore, because it is noticeably dissociated in aqueous solution:

In this chart you can see a lot. The dilution is carried out on the x-axis (x=2 corresponds to c=10 ̄2 mol/l=0.01 mol/l), the black curve is the pH for chloroacetic acid, the white curve is the derivative thereof and the orange is the pH value which is obtained with the formula for weak acids, and the background colors indicate which proportions of free chloroacetic acid (red) and chloroacetate (red) are present.

You clearly see that chloroacetic acid is already distinctly dissociated in relatively concentrated solution (blue content), and at c <0.001 mol/l it is practically a strong acid, i.e. completely dissociated; the formula for weak acids therefore only functions at c>0.01 mol/l. It is therefore fair to say that chloroacetic acid falls within the limit between strong and weak; for example, it is not possible to use as a buffer substance in a conventional manner, or more precisely only at c>0.001 mol/l.

For comparison there is the same graph for acetic acid. You see that acetic acid is also largely strong at very high dilution, i.e. completely dissociated, but this only happens at c≪0.0001 mol/l. That is why it is justified to call acetic acid “weak” — who has ever been dealing with acetic acid solutions in concentration 10 ̄5 mol/l?