Final answer:
The calculation requires the use of the Henderson-Hasselbalch equation. To create a buffer solution of pH 4.94 from a 0.25M acetic acid solution, 0.198125 moles of sodium acetate must be added to 500 mL of the solution.
Step-by-step explanation:
To calculate how many moles of sodium acetate must be added to 500ml of 0.25M acetic acid solution to produce a buffer with a pH of 4.94, the Henderson-Hasselbalch equation is used.
This equation is pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the base form (acetate ion) and [HA] is the concentration of the acid form (acetic acid).
Given the pKa of acetic acid is 4.74 and the desired pH is 4.94, we plug these values into the equation to find the ratio of [A-]/[HA]:
4.94 = 4.74 + log([A-]/[0.25])
solving for [A-], we find that [A-]/[HA] must be 1.585.
Since [HA] is 0.25 M, we can calculate the required concentration of [A-] as: 0.25 M * 1.585 = 0.39625 M for [A-].
The volume of the solution is 500 mL (or 0.5 L), so we need 0.39625 M * 0.5 L = 0.198125 moles of sodium acetate to achieve the desired pH buffer.