Why chloroacetic acid is stronger acid than acetic acid ?

Explanation

The ionisation of acetic acid and chloroacetic acid can be represented as follow :

Chlorine atom has -I effect which :
(a) increases the ionisation of chloroacetic acid.
(b) stabilizes the chloroacetate ion by dispersal of negative charge.
In acetic acid, methyl groups due to its +I effect destabilizes the acetate ion by intensification of negative charge. Hence chloroacetic acid dissociates to a gerater extent than acetic acid and is more acidic than acetic acid.