We assess the strength of an acid on the basis of the completion of the protonolysis reaction as shown........
#HX(aq)+H_2O(l)rightleftharpoonsH_3O^+ + X^-#
For strong acids, i.e. #HX(X!=F); H_2SO_4;HClO_4;HNO_3# this equilibrium lies STRONGLY to the right as written.........and at equilibrium they are usually considered to be stoichiometric in #H_3O^+# and #X^-#.
On the other hand, for weak acids, i.e. #HX=HF;HO_2C-CH_3;# #HNO_2;H_3PO_4#, the equilibrium tends to lie towards the right, and there are quantities of undissociated, free acid at equilibrium.
Now given that #pH=-log_10[H_3O^+]#, the #pH# of a STRONG acids should thus be more negative than the #pH# of a WEAK acid of equivalent concentration.
