Chemical EquilibriaHard
pH of a weak acid
Calculate the pH of 0.100 M acetic acid (CH₃COOH, Ka = 1.8×10⁻⁵).
CH₃COOH ⇌ CH₃COO⁻ + H⁺.
CH₃COOH ⇌ CH₃COO⁻ + H⁺.
Given data
[HA]₀ = 0.100 MK<sub>a</sub> = 1.8×10⁻⁵- Write Ka expression for acetic acid.
- Approximate x = [H⁺] at equilibrium. Ka ≈ x²/0.100. Find x.
- Calculate pH = -log₁₀([H⁺]).
Full worked solution
- Write Ka expression for acetic acid.K_a = [H⁺][A⁻]/[HA] = 1.8×10⁻⁵.
- Approximate x = [H⁺] at equilibrium. Ka ≈ x²/0.100. Find x.x = √(1.8×10⁻⁶) = 1.34×10⁻³ M.
- Calculate pH = -log₁₀([H⁺]).pH = -log(1.34×10⁻³) = 2.87.
Result:pH = 2.87 (weak acid).