Final answer:
To prepare a buffer with a pH of 4.90 using acetic acid and sodium acetate, the acid-salt molar ratio closest to the correct ratio is approximately 1:1 (option A), though an exact calculation yields a slightly different value (closer to 3:2).
Step-by-step explanation:
The question pertains to the preparation of a buffer solution with a specific pH using acetic acid and sodium acetate. According to the Henderson-Hasselbalch equation, for a buffer solution consisting of a weak acid and its conjugate base, pH is given by pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base and [HA] is the concentration of the weak acid. The pKa of acetic acid is 4.74, which is close to the desired pH of 4.90. Thus, to achieve a pH of 4.90, the ratio of [A-] to [HA] should be obtained from rearranging the Henderson-Hasselbalch equation: 4.90 = 4.74 + log([A-]/[HA]), leading to log([A-]/[HA]) = 0.16, and hence [A-]/[HA] = antilog(0.16) = 1.45. This suggests that the ratio should be close to 3:2 (conjugate base to acid), which is a mixture not precisely captured by the given options A) to D). The best approach for the student is to find the exact ratio of conjugate base to acid that corresponds to a value as close as possible to 1.45, which will be closest to option A) 1:1, although a slight excess of the sodium acetate (the conjugate base) would be more precise.