Differing extent of reaction for the two members of the conjugate pair

Depending on position of the pK_a for a weak acid-base conjugate pair in the range 3-12,
the strength of the acid of the pair will be higher than the base or vice versa.

For pairs with higher pK_a, the base of the pair is stronger and reacts to a greater extent than the acid, providing they are in the same concentration.

For pairs with lower pK_a, the acid of the pair is stronger and reacts to a greater extent than the base, providing they are in the same concentration.

The relative extent of reaction of each member is reflected in the extent to which the pH of their solution differs from 7, provided that the concentrations are the same.

Thus for the CH₃CO₂H/CH₃CO₂^– conjugate pair (pK_a 4.7) the acid is stronger.

For 0.1 mol L^–1 CH₃CO₂H, pH = 2.9 (4.1 units lower than 7)
For 0.1 mol L^–1 CH₃CO₂Na, pH = 8.9 (1.9 units higher than 7)

For the NH₄⁺/NH₃ conjugate pair (pK_a 9.2) the base is stronger.

For 0.1 mol L^–1 NH₄⁺, pH = 5.1 (1.9 units lower than 7)
For 0.1 mol L^–1 NH₃, pH = 11.1 (4.1 units higher than 7)