• 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