5.5.5 Buffer Calculations

• The pH of a buffer solution can be calculated using:
  • The K_a of the weak acid
  • The equilibrium concentration of the weak acid and its conjugate base (salt)

• To determine the pH, the concentration of hydrogen ions is needed which can be found using the equilibrium expression

• To simplify the calculations, logarithms are used such that the expression becomes:

• Since -log₁₀ [H⁺] = pH, the expression can also be rewritten as:

Worked Example: Calculating the pH of a buffer solution

Answer

Ethanoic acid is a weak acid that ionises as follows:

CH₃COOH (aq) ⇌ H⁺ (aq) + CH₃COO^- (aq)

• Step 1: Write down the equilibrium expression to find K_a

• Step 2: Rearrange the equation to find [H⁺]

• Step 3: Substitute the values into the expression

= 8.39 x 10^-6 mol dm^-3

• Step 4: Calculate the pH

pH = - log [H⁺]

= -log 8.39 x 10^-6

= 5.08