#HX + H_2O rightleftharpoons H_3O^+ + X^-#
For weak acids, e.g. #HOAc#, #H_2SO_3#, #HF#, the equilibrium lies to the left............
And for strong acids, #H_2SO_4#, #HClO_4#, #HX (X!=F)#, #HNO_3# the equilibrium lies to the right, such that at equilibrium #[HX]# is close to zero, and the aqueous solution is stoichiometric in #H_3O^+#. When you use these in calculations, these have #pK_a# values that are #<2-3#, and ionization may be regarded as complete (even for the diacid).
Because this is an experimental phenomenon, some of these #"strong and weak acids"# have to be committed to memory. The practical component of your course will help you remember.
