Calculate the pH of a buffer solution?

[CH₃COO⁻] = (0.1 mol) / (1 dm³) = 1 M
Hence, [CH₃COO⁻]/[CH₃COOH] = (1 M) / (1 M) = 1

Consider the dissociation of CH₃COOH:
CH₃COOH(aq) + H₂O(ℓ) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)    Ka = 1.8 × 10⁻⁵

Henderson-Hasselbalch equation:
pH = pKa + log([CH₃COO⁻]/[CH₃COOH])
pH = -log(1.8 × 10⁻⁵) + log(1)
pH = 4.74