Practical Example Calculating Buffer pH
This section provides a practical example of how to calculate the pH of a buffer solution, which is a common type of question in A Level Chemistry buffer calculations.
Example Ethanoic acid buffer
Given
- 125 cm³ of 0.8 mol dm⁻³ ethanoic acid (CH3COOH)
- 75 cm³ of 1 mol dm⁻³ sodium ethanoate (CH3COONa)
- Ka of ethanoic acid = 1.74 × 10⁻⁵ mol dm⁻³
Step 1 Calculate new concentrations after mixing
CH3COOH = 0.8×125 / 200 = 0.5 mol dm⁻³
CH3COO− = 1×75 / 200 = 0.375 mol dm⁻³
Step 2 Use the equation H+ = Ka × HA / A−
H+ = 1.74×10−5×0.5 / 0.375 = 2.32 × 10⁻⁵ mol dm⁻³
Step 3 Calculate pH
pH = -log2.32×10−5 = 4.63
Highlight This example demonstrates how to prepare a buffer solution with a specific pH and how to calculate pH of buffer solution given molarity and volume.
Example If you needed to prepare a pH 4 buffer solution, you would adjust the ratio of acid to conjugate base to achieve this pH.
Understanding these calculations is crucial for mastering buffers in A Level Chemistry OCR and other exam boards.