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Acid Ionization Constants ( K a ). Strong acids like HCl ( aq ) dissociate (or ionize) fully (nearly 100%) into ions. Eg . HCl ( aq ) + H 2 O (l)  H 3 O + ( aq ) + Cl - ( aq ) Ka = [ H 3 O + ][ Cl - ] where K a is a large value

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Acid ionization constants k a
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 k b
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


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