Summary equal amounts

When the concentrations of the acid and base of the conjugate pair are equal (irrespective of whether they are both large or both small), at equilibrium
 
[H3O+] equals Ka.
pH equals pKa

because, as shown, the concentrations cancel
 
Ka = [H3O+] × [conjugate base]
[acid]
 
Ka = [H3O+] × [conjugate base]
[acid]
Dr Sheila Woodgate
© The University of Auckland 2002