Virginia HESI Level 4 Practice Test

A buffer works by having a weak acid and its conjugate base in the solution. This pairing provides both a source of protons and a sink for protons, which lets the solution resist pH changes when small amounts of acid or base are added. When a little strong acid is added, the conjugate base captures the extra H+, forming the weak acid and preventing a large drop in pH. When a little strong base is added, the weak acid donates H+ to neutralize it, preventing a large rise in pH. The buffering effect is strongest near the weak acid’s pKa, where the amounts of acid and conjugate base are comparable, described by pH = pKa + log([A-]/[HA]). A salt of a weak acid and a strong base would create a basic solution but doesn’t provide the ongoing acid–base exchange needed for buffering. Water alone doesn’t resist pH changes. A strong acid with its conjugate base doesn’t buffer well because the strong acid fully dissociates and its conjugate base is too weak to balance pH changes.