Final answer:
The most effective buffer pair from the given options is 0.10 M HA and 0.10 M A−, as it provides a 1:1 ratio of weak acid to its conjugate base, allowing for maximum pH control.
Step-by-step explanation:
The student's question pertains to the effectiveness of buffers at different concentrations of a weak acid (HA) and its conjugate base (A-). A buffer composed of roughly equal concentrations of HA and A− is most effective at controlling pH. Taking into account the Henderson-Hasselbalch Equation, a 1:1 ratio of HA to A− would result in the greatest buffer capacity, providing maximum pH control, especially when the buffer concentration is high. This conforms with the advice to choose a buffer system where the pKa is close to the pH one aims to maintain.
Among the options given, the pair with an equal concentration of 0.10 M HA and 0.10 M A− creates an effective buffer, as it falls under the ideal 1:1 ratio. Adding 0.001 M HCl to this solution would lead to a buffer adjusted to concentrations of 0.101 M HA and 0.099 M A−. Per the Henderson-Hasselbalch Equation, pH changes minimally here, given that the ratio of [A−]/[HA] remains nearly identical at around 1. Hence, this pair would allow the solution to maintain a consistent pH when an acid or base is added.