You have 500.0 mL of a buffer solution containing 0.30 M acetic acid (CH₃COOH) and 0.20 M sodium acetate (CH₃COONa). What will the pH of this solution be after the addition of 20.0 mL of 1.00 M NaOH solution [K_a = 1.8 * 10^-5]

Question

Quizplus · Accepted Answer

1. Write the equation for the reaction between acetic acid and sodium hydroxide:

CH3COOH + NaOH → CH3COONa + H2O

2. Write the expression for the Ka of acetic acid:

Ka = [H+][CH3COO-]/[CH3COOH]

3. Write the equation for the dissociation of acetic acid in water:

CH3COOH + H2O ⇌ H3O+ + CH3COO-

4. Write the expression for the pH of the buffer solution:

pH = pKa + log([CH3COO-]/[CH3COOH])

5. Calculate the initial concentrations of acetic acid and acetate ion:

[CH3COOH] = 0.30 M
[CH3COO-] = 0.20 M

6. Calculate the initial pH of the buffer solution:

pKa of acetic acid = 4.76
pH = 4.76 + log(0.20/0.30) = 4.65

7. Calculate the new concentrations of acetic acid and acetate ion after the addition of NaOH:

[CH3COOH] = 0.30 M - (20.0 mL × 1.00 mol/L × 1 L/1000 mL) = 0.28 M
[CH3COO-] = 0.20 M + (20.0 mL × 1.00 mol/L × 1 L/1000 mL) = 0.22 M

8. Calculate the new pH of the buffer solution using the Henderson-Hasselbalch equation:

pH = pKa + log([CH3COO-]/[CH3COOH])
pH = 4.76 + log(0.22/0.28) = 4.71

Therefore, the answer is choice B.

You Have 500