The dissociation constant of a weak acid HA and weak base BOH are 2×10-5 and 5×10-6 respectively.The equilibrium constant for the neutralization reaction of the two is ( ignore hydrolysis of resulting salt)

Dissociation constant for weak acid (Ka) = 2 x 10-5 
Dissociation constant for weak base (Kb) =  5 x 10-6  
Consider the weak acid can be represented as HA and weak base can be represented as BOH.
Neutralisation reaction:  HA + BOH ⇌​ A- + BH+
                       HA +H2O ⇌​ A- + H3O+        Ka
                      BOH + H2O ⇌​ BH+ + OH-   Kb
                      H3O+ + OH- ⇌​ 2H2O            1/Kw    (Kw is tthe dissociation constant of water)
                      _______________________________
Net Reaction: HA + BOH ⇌​ A- + BH+
Thus the equilibrium constant of the neutralisation reaction  will be the product of the equilibrium constant of the above three reactions and it can be written as 
K = KaKbKw


Equilibrium constant for the neutralisation reaction (K) = KaKbKw =2×10-5×5×10-610-14=104

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