Which pair forms a buffer solution?

• A. Acetic acid and acetate
• B. Hydrochloric acid and chloride
• C. Ammonia and ammonium
• D. Nitric acid and nitrate

Answer: (A)

Buffers resist changes in pH because they contain a weak acid and its conjugate base in appreciable amounts. In the pair acetic acid and acetate, you have a weak acid (acetic acid) and its conjugate base (acetate) present together. This setup lets the solution neutralize both added base and added acid: added base gets neutralized by converting acetic acid to acetate, and added acid gets neutralized by converting acetate back to acetic acid. The buffering action keeps the pH near the acid’s pKa, about 4.76 for acetic acid, which is where such a system works best.

Strong acids like hydrochloric acid and nitric acid dissociate completely, so their conjugate bases (chloride and nitrate) are too weak to absorb added protons or hydroxide ions effectively. That’s why those pairs don’t form buffers. Ammonia and ammonium can also form a buffer, since it’s a weak base with its conjugate acid, but the classic and most straightforward example highlighted here is the weak acid and its conjugate base pair.