Suppose I want to make a buffer solution in any of the three ways possible with dihydrogen phosphate as my weak acid. In the first way, a certain ratio of the weak acid and its conjugate base are added at once to form the buffer. Why does this not return to the equilibrium described by its Ka value instead of forming a stable buffer? With a pka of 7.2, shouldn't the mixture shift to the weak acid to reestablish equilibrium. The other two ways involve adding a strong acid or base to a weak base or weak acid respectively, but I still see the same issue. Say a strong base is added to a solution of the dihydrogen phosphate and the same moles added react and form an equal number of moles of the conjugate base. After this is complete, why doesn't equilibrium get restored. Is the weak acid/base equilibrium out-competed by the seperate strong base/weak acid to weak acid/weak base equilibrium?
Hope this made sense. Thanks in advance!