Common Ion Effect
CH3CO2H + H2O ? CH3CO2- + H3O+
Adding more CH3CO2- to the solution shifts the equilibrium to the left, making the solution less acidic (higher pH).
0.100 M CH3CO2H pH = 2.879
0.100 M CH3CO2H,
0.050 M NaCH3CO2 pH = 4.456
CH3CO2- is an ion common to CH3CO2H and NaCH3CO2 solutions, consistent with LeChatelier’s Principle