An acetic acid buffer containing 0.50 M CH₃COOH and 0.50 M CH₃COONa has a pH of 4.74. What will the pH be after 0.0020 mol of HCl has been added to 100.0 mL of the buffer? A) 4.77 B) 4.71 C) 4.68 D) 4.62 E) none of the above

Question

Quizplus · Accepted Answer

First, we need to determine the initial concentrations of CH3COOH and CH3COO- in the buffer solution. Since the total concentration of the buffer is 0.50 M and the ratio of CH3COOH to CH3COO- is 1:1, the initial concentration of each can be calculated as follows:

[CH3COOH] = 0.50 M × 0.50 = 0.25 M
[CH3COO-] = 0.50 M × 0.50 = 0.25 M

Next, we need to determine the reaction that occurs when the HCl is added to the buffer solution. The H+ ions from the HCl will react with the CH3COO- ions to form more CH3COOH according to the equation:

CH3COO- + H+ → CH3COOH

Since the initial concentrations of CH3COOH and CH3COO- are equal, half of the H+ ions will react with CH3COO- to form CH3COOH and the other half will remain in solution. We can use the Henderson-Hasselbalch equation to determine the new pH of the buffer solution:

pH = pKa + log([A-]/[HA])

where pKa is the dissociation constant of acetic acid (4.76), [A-] is the concentration of CH3COO- after the addition of HCl, and [HA] is the concentration of CH3COOH after the addition of HCl.

We can calculate the new concentrations of CH3COOH and CH3COO- as follows:

[CH3COOH] = 0.25 M + 0.001 M = 0.251 M
[CH3COO-] = 0.25 M - 0.001 M = 0.249 M

Using these values, we can calculate the new pH of the buffer solution:

pH = 4.76 + log(0.249/0.251)
pH = 4.71

Therefore, the pH of the buffer solution after the addition of 0.0020 mol of HCl is 4.71, so the correct answer is B.

An Acetic Acid Buffer Containing 0