Final answer:
The amount of HCl needed to adjust the pH of a buffer solution depends on the buffer's composition and capacity.
Step-by-step explanation:
To determine how much HCl must be added to a liter of buffer, it's essential to consider the buffer's capacity and the desired pH change. As illustrated in the given examples, buffers resist changes in pH when acids or bases are added due to the presence of a weak acid or base and its conjugate salt.
In an acetic acid-sodium acetate buffer, adding 0.020 mol HCl to 1 liter only changes the pH minutely from 4.74 to 4.70, showing the buffer's capacity to resist pH change. In contrast, unbuffered water experiences a dramatic pH shift with the same amount of HCl or NaOH.
Thus, the quantity of HCl needed to achieve a specific pH level in a buffer solution will depend on the buffer's composition and its buffering capacity.