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As indicated in http://catalog. flatworldknowledge. com/bookhub/reader/2547 - gob-ch10_s04, weak acids are relatively common, even in the foods we eat. But we occasionally come across a strong acid or base, such as stomach acid, that has a strongly acidic pH of 12. By definition, strong acids and bases can produce a relatively large amount of hydrogen or hydroxide ions and, as a consequence, have a marked chemical activity. In addition, very small amounts of strong acids and bases can change the pH of a solution very quickly. If 1 mL of stomach acid [which we will approximate as 0.05 M HCl aq ] is added to the bloodstream, and if no correcting mechanism is present, the pH of the blood would go from about 7.4 to about 4.9a pH that is not conducive to continued living. Fortunately, the body has a mechanism for minimizing such dramatic pH changes. The mechanism involves a buffer, a solution that resists dramatic changes in pH. Buffers do so by being composed of certain pairs of solutes: either a weak acid plus a salt derived from that weak acid or a weak base plus a salt of that weak base. For example, a buffer can be composed of dissolved acetic acid HC2H3O2, a weak acid and sodium acetate NaC2H3O2, a salt derived from that acid . Another example of a buffer is a solution containing ammonia NH3, a weak base and ammonium chloride NH4Cl, a salt derived from that base . Let us use an acetic acidsodium acetate buffer to demonstrate how buffers work. If a strong basea source of OH aq ionsis added to the buffer solution, those hydroxide ions will react with the acetic acid in an acid-base reaction: HC2H3O2 aq + OH aq H2O + C2H3O2 aq Rather than changing the pH dramatically by making the solution basic, the added hydroxide ions react to make water, and the pH does not change much.