then when concentration of hydronium is [H3O+]= 1 M, pH = - log [1] = -0 = 0 which is when the liquid has nothing else than H3O+ ions inside, or H2O molecules (which is water) coupled with the same number of H+ ions (hydrogen ions). As I stated 3 years ago, pH =0 is considered formally for lay reasons as 100% acid.
Similarly when the hydronium concentration is 0.00000000000001 M (1 at the 14th decimal slot behind the decimal point), pH will be pH = - log [H3O+] = - log [0.00000000000001] = - (-14) = 14 ... considered formally as 100% base. Such that OH- ions (hydroxyl ions) outnumber 10 Quadrillion times the H+ ions, theoretically, same as one H+ ion per 10 000 000 000 000 OH- which is supposed to be 100% base.