Kₐ for CH₃ COOH is 1.8 × 10⁻⁵ and K_b for NH₄ OH is 1.8 × 10⁻⁵. The pH of ammonium acetate solution will be _________.

Solution

Chemistry ma’am · 3 steps

Both ions hydrolyse equally here, so the acid made by one exactly cancels the base made by the other.

1. Both ions hydrolyse. Ammonium acetate is a salt of a weak acid and a weak base. NH₄⁺ hydrolyses to give H⁺, and CH₃COO⁻ hydrolyses to give OH⁻. The pH depends on which of the two hydrolyses more.

   pH = 7 + 1/2(pKₐ - pK_b)

2. Put in numbers. Kₐ = K_b = 1.8 × 10⁻⁵, so pKₐ = pK_b = 4.74. The bracket is 4.74 - 4.74 = 0. The salt concentration does not appear in the formula, so you do not need it.
3. So. pH = 7 + 0 = 7. The solution is neutral.

The idea to keep. For a salt of a weak acid and a weak base, pH = 7 + 1/2(pKₐ - pK_b) and concentration drops out. Equal Kₐ and K_b always gives pH 7.

Answer from the NTA final answer key. Solution by PrepWiser.