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Acid Ionization Constants (Ka) Strong acids like HCl(aq) dissociate (or ionize) fully (nearly 100%) into ions. Eg. HCl(aq) + H2O(l) H3O+(aq) + Cl-(aq) Ka = [H3O+][Cl-] where Ka is a large value [HCl] (lots of products) And pH = -log[H3O+] directly from strong acid initial concentration *only strong acids are: HCl, HI, HBr, HNO3, H2SO4, HClO4
Weak acids like CH3COOH(aq) do not dissociate fully into ions (only 5%). Eg. CH3COOH (aq) + H2O(l) H3O+(aq) + CH3COO-(aq) Ka =[CH3COO-][H3O+] where Ka is a small value [CH3COOH ] (very few products) pH can not be found directly from the weak acid initialconcentration. An ICE chart must be used to find out how much [H3O+] is actually formed and then the formula: pH = -log[H3O+] can be used.
Base Ionization Constants (Kb) Calculations for base ionization constants are exactly the same as for the acid constants except the equilibrium equation and expression contains OH- instead of H3O+ terms. Eg. B(aq) + H2O(l) BH+(aq) + OH-(aq) Kb = [BH+][OH-] [B] Strong bases like NaOH(aq) dissociate (or ionize) fully (nearly 100%) into ions. pOH = -log[OH-] directly from strong base initialconcentration *only strong bases are: Family I and some Family II hydroxides
Weak bases like NH3(aq) do not dissociate fully into ions (only 5%). pOHcan not be found directly from the weak base initial concentration. An ICE chart must be used to find out how much [OH-] is actually formed and then the formula: pOH = -log[OH-] can be used. Ka and Kb are related by the relationship: Ka*Kb = Kw