A 0.10 M solution of ammonium acetate (NH4+ and AcO-), and why?

A 0.10 M solution of ammonium acetate (NH4+ and AcO-), and why?

• A. 7.00
• B. 5.00
• C. 8.50
• D. 6.50

Answer: (A)

A salt formed from a weak acid and a weak base tends to be near neutral when the strengths of the conjugate acid and conjugate base are similar. In ammonium acetate, NH4+ acts as a weak acid with Ka around 5.6×10^-10, and AcO^- is a weak base with Kb = Kw/Ka of acetic acid ≈ 1×10^-14 / 1.8×10^-5 ≈ 5.6×10^-10. These values are effectively equal, so the tendency to produce H+ from NH4+ and OH- from AcO- cancels out, pulling the pH to about 7. Using the approximate relation pH ≈ 7 + 1/2 log(Kb/Ka) gives pH ≈ 7 since Kb ≈ Ka. The 0.10 M concentration doesn’t shift this significantly, so the solution is essentially neutral at about 7.00.

A salt formed from a weak acid and a weak base tends to be near neutral when the strengths of the conjugate acid and conjugate base are similar. In ammonium acetate, NH4+ acts as a weak acid with Ka around 5.6×10^-10, and AcO^- is a weak base with Kb = Kw/Ka of acetic acid ≈ 1×10^-14 / 1.8×10^-5 ≈ 5.6×10^-10. These values are effectively equal, so the tendency to produce H+ from NH4+ and OH- from AcO- cancels out, pulling the pH to about 7. Using the approximate relation pH ≈ 7 + 1/2 log(Kb/Ka) gives pH ≈ 7 since Kb ≈ Ka. The 0.10 M concentration doesn’t shift this significantly, so the solution is essentially neutral at about 7.00.