Final answer:
A buffer solution at a pH of 8.5 is more effective at buffering the addition of a strong base than a strong or weak acid. In this case, it has more capacity to buffer against NaOH, as it can neutralize the added hydroxide ions without significantly changing the pH.
Step-by-step explanation:
The question concerns which substance a buffer solution with a pH of 8.5 and a pK of 9.5 would have more capacity to buffer against. A buffer is most effective when the pH is within one unit of its pK value. Since the pH of the solution is closer to the pK, it means the buffer contains similar amounts of the weak acid and its conjugate base, which allows it to effectively neutralize added acids. Therefore, a buffer at a pH of 8.5 would have more capacity to buffer the addition of NaOH (a strong base) than HCl (strong acid), Na2CO3 (a weak base), or H2SO4 (strong acid), because the addition of a base like NaOH would be countered by the weak acid present in the buffer solution.
For example, adding 10.0 mL of 1.0 M NaOH to 1.0 L of a similar buffer would only cause a minor increase in pH, demonstrating the buffer's capacity to take up additional hydroxide ions. This principle is reflected in the diminished change in pH when increasing amounts of NaOH are added to a buffer solution, especially one with concentrations of at least 0.500 M.