Question

1 A buffer is prepared by mixing 50.0 mL of 0.100 M acetic acid with 30.0 mL of 0.100 M sodium hydroxide and 20.0 mL of distilled water. Estimate the concentration of acetic acid in the resulting buffer. Report your answer in moles/liter, but do not include units with your answer. 2 Determine the concentration of acetate ions in the buffer solution. Report your answer in moles per liter, but do not include units in your answer. 3 Estimate the pH of the buffer solution using the Henderson-Hasselbalch equation. 4 Using the Henderson-Hasselbalch equation, estimate the pH of the buffer if an additional 10.0 mL of 0.100 M sodium hydroxide is added. 5 Estimate the pH of the original buffer if 10.0 mL of 0.100 M hydrochloric acid is added.

Answer 1

1)

concentration of acetic acid = 50.0 x 0.100 / 50 + 30 = 0.0625 M

concentration of NaOH = 30.0 x 0.100 / 50 + 30 = 0.0375

CH3COOH (aq) +   NaOH (aq) -----------> CH3COONa (aq) + H2O (l)

0.0625                    0.0375                                0                         0

0.025                       0                                    0.0375

concentration of acetic acid = 0.0250 M

2)

concentration of acetate ions = 0.0375 M

3)

pH = pKa + log [salt / acid]

    = 4.74 + log [0.0375 / 0.0250]

pH = 4.92

4)

millimoles of NaOH = 10 x 0.1 = 1

pH = pKa + log [salt + C / acid - C]

    = 4.74 + log [3 + 1 / 2 - 1]

pH = 5.34

5)

pH = pKa + log [salt - C / acid + C]

    = 4.74 + log [3 - 1 / 2 + 1]

pH = 4.56