ACS Bezique Events Practice Test

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What is the purpose of a buffer and why does pH remain relatively constant upon addition of small amounts of strong acid or base?

Buffer resists pH change by neutralizing added acid or base via conjugate pairs; pH remains near pKa as described by Henderson-Hasselbalch Buffer only works at the exact pH value of the solution and cannot buffer outside that Buffer raises pH by consuming base but not acid Buffer prevents any change in hydrogen ion activity regardless of amount added

Correct Answer:

Buffer resists pH change by neutralizing added acid or base via conjugate pairs; pH remains near pKa as described by Henderson-Hasselbalch

Explanation:

A buffer works by using a conjugate acid–base pair to neutralize added acid or base. When a small amount of strong acid is added, the conjugate base (A−) in the buffer reacts with the extra H+ to form the weak acid (HA): A− + H+ → HA. This removes some of the added protons, so the pH rises only a little. When a small amount of strong base is added, the weak acid (HA) donates a proton to the hydroxide: OH− + HA → H2O + A−, forming more of the conjugate base and again limiting the pH change. The Henderson–Hasselbalch expression pH = pKa + log([A−]/[HA]) shows why the pH stays near the pKa: small additions shift the ratio [A−]/[HA] only slightly, keeping pH in a narrow range around the pKa (the buffer range). A few caveats: buffers have finite capacity, so large additions can overwhelm them and cause bigger pH changes; and buffers don’t fix pH absolutely or provide resistance in one direction only.

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