If a small amount of a strong base is added to a buffer made up of a weak acid, HA, and the sodium salt of its conjugate base, NaA, the pH of the buffer solution does not change appreciably because

Question

BlackTom AI · Accepted Answer

The scenario involves a buffer made of a weak acid HA and its conjugate base A- (from NaA). When a small amount of a strong base is added, the key reaction occurs between the base and the weak acid component, not with A- itself, which explains why pH stays relatively constant.
Option 1: 'the strong base reacts with HA to give A—.' This is correct because OH− from the strong base deprotonates HA, forming A− and H2O. This consumes some HA but creates more A−, which buffers the pH via the HA/A− equilibrium and minimizes pH change.
Option 2: 'the strong base reacts with A— to give HA.' This would imply converting conjugate base back to the acid, which is not how a buffering action operates when a small amount of strong base is added; in fact, the reaction goes the other way (HA + OH− → A− + H2O).
Option 3: 'the Ka of HA is changed.' The acid dissociation constant Ka for HA is an intrinsic property and does not change due to adding a base; buffering affects pH through species ratios, not the fundamental Ka value.
Option 4: 'no reaction occurs.' This is incorrect because OH− neutralizes HA, producing water and A−, a reaction that actively consumes HA and forms A−, contributing to the buffer’s resilience to pH change.
Option 5: 'the strong base reacts with A— to give H2O.' While water is a product in the overall reaction, the primary interaction in forming the buffer shift is the base reacting with HA, not directly with A−; this option misstates the main chemical event and omits the formation of additional A− from HA.

If a small amount of a strong base is added to a buffer made up of a weak acid, HA, and the sodium salt of its conjugate base, NaA, the pH of the buffer solution does not change appreciably because单项选择题

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