[derek0o0]

why some acid is stronger than the other?
like HCl and Acetic acid

[Ak]

it usually depends on how stable the conjugate base.  The strength of an acid depends on how easily it can donate a hydrogen ion or the stability of the remaining negative ion after removing the hydrogen ion.  How easily it donates a hydrogen depends on hos stable the negative ion is, so they go hand in hand.
This is were Ka's come from as Ka values are good indicators of the ability to ionize