These bar graphs show the concentration of hydrogen ions and
hydroxide water in pure water and the concentration all the species
in a solution that contains 0.10 M acetic and and 0.10 M sodium acetate.

The pH of water is 7 and the pH of the mixture of acetic acid and
sodium acetate is 4.7. Let's add a drop of acetic acid and see what
happens to the pH of each solution.