Strong and Weak Acids and Acid Ionization Constant (Ka ) Strong and Weak Acids and Acid Ionization Constant Acids are classified as either strong or weak, based on their ionization in water. Astrong acidis an acid which is completely ionized in an aqueous solution. Hydrogen chloride (HCl) ionizes completely into hydrogen ions and chloride ions in water. Aweak acidis an acid that ionizes only slightly in an aqueous solution. Acetic acid (found in vinegar) is a very common weak acid. Its ionization is shown below. The ionization of acetic acid is incomplete, and so the equation is shown with a double arrow. The extent of ionization of weak acids varies, but is generally less than 10%. A 0.10 M solution of acetic acid is only about 1.3% ionized, meaning that the equilibrium strongly favors the reactants. Weak acids, like strong acids, ionize to yield the H+ ion and a conjugate base. Because HCl is a strong acid, its conjugate base (Cl−) is extremely weak. The chloride ion is incapable of accepting the H+ion and becoming HCl again. In general, the stronger the acid, the weaker its conjugate base. Likewise, the weaker the acid, the Relative Strengths of Acids and their Conjugate Bases AcidConjugate Base Strong Acids HCl (hydrochloric acid) (strongest)Cl− (chloride ion) (weakest) H2SO 4
